<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with WebFilings-->
<!-- -->
<link:linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <link:roleRef roleURI="http://www.regeneron.com/role/CollaborationAndContractResearchAgreementsDetails" xlink:href="regn-20121231.xsd#CollaborationAndContractResearchAgreementsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/CommitmentsAndContingenciesCapitalLeasesDetails" xlink:href="regn-20121231.xsd#CommitmentsAndContingenciesCapitalLeasesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/CommitmentsAndContingenciesFacilityLeasesDetails" xlink:href="regn-20121231.xsd#CommitmentsAndContingenciesFacilityLeasesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/CommitmentsAndContingenciesOperatingLeasesDetails" xlink:href="regn-20121231.xsd#CommitmentsAndContingenciesOperatingLeasesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/CommitmentsAndContingenciesResearchCollaborationAndLicensingAgreementsDetails" xlink:href="regn-20121231.xsd#CommitmentsAndContingenciesResearchCollaborationAndLicensingAgreementsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/ConsolidatedBalanceSheets" xlink:href="regn-20121231.xsd#ConsolidatedBalanceSheets" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:href="regn-20121231.xsd#ConsolidatedBalanceSheetsParenthetical" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/ConsolidatedStatementsOfCashFlows" xlink:href="regn-20121231.xsd#ConsolidatedStatementsOfCashFlows" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveIncomeLoss" xlink:href="regn-20121231.xsd#ConsolidatedStatementsOfOperationsAndComprehensiveIncomeLoss" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/ConsolidatedStatementsOfStockholdersEquity" xlink:href="regn-20121231.xsd#ConsolidatedStatementsOfStockholdersEquity" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/ConvertibleDebtDetails" xlink:href="regn-20121231.xsd#ConvertibleDebtDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/DeferredRevenueDetails" xlink:href="regn-20121231.xsd#DeferredRevenueDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/DocumentAndEntityInformation" xlink:href="regn-20121231.xsd#DocumentAndEntityInformation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/EmployeeSavingsPlanDetails" xlink:href="regn-20121231.xsd#EmployeeSavingsPlanDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/ExecutiveStockPurchasePlanDetails" xlink:href="regn-20121231.xsd#ExecutiveStockPurchasePlanDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/FairValueMeasurementsDetails" xlink:href="regn-20121231.xsd#FairValueMeasurementsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/IncomeTaxesDetails" xlink:href="regn-20121231.xsd#IncomeTaxesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/LegalMattersDetails" xlink:href="regn-20121231.xsd#LegalMattersDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/LongTermIncentivePlansDetails" xlink:href="regn-20121231.xsd#LongTermIncentivePlansDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/MarketableSecuritiesDetails" xlink:href="regn-20121231.xsd#MarketableSecuritiesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/NetIncomeLossPerShareDetails" xlink:href="regn-20121231.xsd#NetIncomeLossPerShareDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/NetProductSalesDetails" xlink:href="regn-20121231.xsd#NetProductSalesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/PropertyPlantAndEquipmentDetails" xlink:href="regn-20121231.xsd#PropertyPlantAndEquipmentDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/StockholdersEquityDetails" xlink:href="regn-20121231.xsd#StockholdersEquityDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/SummaryOfSignificantAccountingPoliciesDetails" xlink:href="regn-20121231.xsd#SummaryOfSignificantAccountingPoliciesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.regeneron.com/role/TechnologyLicensingAgreementsDetails" xlink:href="regn-20121231.xsd#TechnologyLicensingAgreementsDetails" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <link:definitionLink xlink:role="http://www.regeneron.com/role/CollaborationAndContractResearchAgreementsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable" xlink:label="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_B167F02401DDE6F67500DA9DB9479975" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_B167F02401DDE6F67500DA9DB9479975" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_18614A5E54ADAF0AA351DA9DB9479D48" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_B167F02401DDE6F67500DA9DB9479975" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_18614A5E54ADAF0AA351DA9DB9479D48" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_8F96AA836CF81F6EA195DA9DB947E708_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_18614A5E54ADAF0AA351DA9DB9479D48" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_8F96AA836CF81F6EA195DA9DB947E708_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_8F96AA836CF81F6EA195DA9DB947E708" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_18614A5E54ADAF0AA351DA9DB9479D48" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_8F96AA836CF81F6EA195DA9DB947E708" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CollaborationAgreementWithSanofiMember" xlink:label="loc_regn_CollaborationAgreementWithSanofiMember_5D933F151FC764382630DA9DB948EC74" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_8F96AA836CF81F6EA195DA9DB947E708" xlink:to="loc_regn_CollaborationAgreementWithSanofiMember_5D933F151FC764382630DA9DB948EC74" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CollaborationAgreementWithBayerHealthcareLlcMember" xlink:label="loc_regn_CollaborationAgreementWithBayerHealthcareLlcMember_260F757A2362690AB877DA9DB9484D5C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_8F96AA836CF81F6EA195DA9DB947E708" xlink:to="loc_regn_CollaborationAgreementWithBayerHealthcareLlcMember_260F757A2362690AB877DA9DB9484D5C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_GrantFromNationalInstitutesOfHealthMember" xlink:label="loc_regn_GrantFromNationalInstitutesOfHealthMember_06D9212A8E5D429DEEA9DA9DB94890A9" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_8F96AA836CF81F6EA195DA9DB947E708" xlink:to="loc_regn_GrantFromNationalInstitutesOfHealthMember_06D9212A8E5D429DEEA9DA9DB94890A9" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductAxis" xlink:label="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductAxis_DACFD5B8B16507B05D60DA9DB948AE86" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_B167F02401DDE6F67500DA9DB9479975" xlink:to="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductAxis_DACFD5B8B16507B05D60DA9DB948AE86" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain" xlink:label="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain_09E76F3AECAE6F3F4B7ADA9DB948C623_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductAxis_DACFD5B8B16507B05D60DA9DB948AE86" xlink:to="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain_09E76F3AECAE6F3F4B7ADA9DB948C623_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain" xlink:label="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain_09E76F3AECAE6F3F4B7ADA9DB948C623" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductAxis_DACFD5B8B16507B05D60DA9DB948AE86" xlink:to="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain_09E76F3AECAE6F3F4B7ADA9DB948C623" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AfliberceptAgreementMember" xlink:label="loc_regn_AfliberceptAgreementMember_6873292A1186ABF58550DA9DB9483751" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain_09E76F3AECAE6F3F4B7ADA9DB948C623" xlink:to="loc_regn_AfliberceptAgreementMember_6873292A1186ABF58550DA9DB9483751" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AntibodyCollaborationMember" xlink:label="loc_regn_AntibodyCollaborationMember_F5818B00EC8956723058DA9DB948BFE3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain_09E76F3AECAE6F3F4B7ADA9DB948C623" xlink:to="loc_regn_AntibodyCollaborationMember_F5818B00EC8956723058DA9DB948BFE3" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_VelocigeneAgreementMember" xlink:label="loc_regn_VelocigeneAgreementMember_14370773EB4AC402CB01DA9DB948CA73" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchAndDevelopmentArrangementContractToPerformForOthersByProductDomain_09E76F3AECAE6F3F4B7ADA9DB948C623" xlink:to="loc_regn_VelocigeneAgreementMember_14370773EB4AC402CB01DA9DB948CA73" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_35085238A7E9F24C090EDA9DB94802AE" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_B167F02401DDE6F67500DA9DB9479975" xlink:to="loc_us-gaap_TypeOfArrangementAxis_35085238A7E9F24C090EDA9DB94802AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_47410D10D28BF9962997DA9DB94887D9_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_35085238A7E9F24C090EDA9DB94802AE" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_47410D10D28BF9962997DA9DB94887D9_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_47410D10D28BF9962997DA9DB94887D9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_35085238A7E9F24C090EDA9DB94802AE" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_47410D10D28BF9962997DA9DB94887D9" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MarketingApprovalMember" xlink:label="loc_regn_MarketingApprovalMember_B735DDB867A7832DE67DDA9DB948151C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_47410D10D28BF9962997DA9DB94887D9" xlink:to="loc_regn_MarketingApprovalMember_B735DDB867A7832DE67DDA9DB948151C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PricingApprovalMember" xlink:label="loc_regn_PricingApprovalMember_ACBD573327CAA3C35F74DA9DB948FE39" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_47410D10D28BF9962997DA9DB94887D9" xlink:to="loc_regn_PricingApprovalMember_ACBD573327CAA3C35F74DA9DB948FE39" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NumberOfSharesOfCommonStockPurchasedInShares" xlink:label="loc_regn_NumberOfSharesOfCommonStockPurchasedInShares_7095F1BE017C4FD03202DA9DB9480D19" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_NumberOfSharesOfCommonStockPurchasedInShares_7095F1BE017C4FD03202DA9DB9480D19" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CompanyIncurredExpensesAssociatedWithSanofiCollaborations" xlink:label="loc_regn_CompanyIncurredExpensesAssociatedWithSanofiCollaborations_41A8FCDE2F889C89CEDDDA9DB9480638" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_CompanyIncurredExpensesAssociatedWithSanofiCollaborations_41A8FCDE2F889C89CEDDDA9DB9480638" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AmountOfNonRefundablePaymentReceivedForCollaborationAgreement" xlink:label="loc_regn_AmountOfNonRefundablePaymentReceivedForCollaborationAgreement_25FB14C41222A61D5F35DA9DB949AB71" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AmountOfNonRefundablePaymentReceivedForCollaborationAgreement_25FB14C41222A61D5F35DA9DB949AB71" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NumberOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased" xlink:label="loc_regn_NumberOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased_0FEE8C1E9B4E862FD324DA9DB949306F" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_NumberOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased_0FEE8C1E9B4E862FD324DA9DB949306F" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CashPriceOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased" xlink:label="loc_regn_CashPriceOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased_4FB8C975CB2470497F2BDA9DB949DC7C" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_CashPriceOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased_4FB8C975CB2470497F2BDA9DB949DC7C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RoyaltyBasedOnPercentageOfAnnualSalesInJapan" xlink:label="loc_regn_RoyaltyBasedOnPercentageOfAnnualSalesInJapan_31538B0F0DC971702A01DA9DB94997AC" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_RoyaltyBasedOnPercentageOfAnnualSalesInJapan_31538B0F0DC971702A01DA9DB94997AC" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedRelatingToMarketingApprovals" xlink:label="loc_regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedRelatingToMarketingApprovals_E9600C993698341242B5DA9DB949DAF3" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedRelatingToMarketingApprovals_E9600C993698341242B5DA9DB949DAF3" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInUSOrEuropeanUnion" xlink:label="loc_regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInUSOrEuropeanUnion_A8C7DCB6154BBC97F12DDA9DB9492DB9" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInUSOrEuropeanUnion_A8C7DCB6154BBC97F12DDA9DB9492DB9" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumNumberOfProductCandidatesForWhichSubstantiveMilestonePaymentsMayBeReceivedInUSOrEuropeanUnion" xlink:label="loc_regn_MaximumNumberOfProductCandidatesForWhichSubstantiveMilestonePaymentsMayBeReceivedInUSOrEuropeanUnion_EA433128C3BDE95F9A73DA9DB9494EC0" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MaximumNumberOfProductCandidatesForWhichSubstantiveMilestonePaymentsMayBeReceivedInUSOrEuropeanUnion_EA433128C3BDE95F9A73DA9DB9494EC0" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInJapan" xlink:label="loc_regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInJapan_F9FDE2548AF2B369E037DA9DB9493A70" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInJapan_F9FDE2548AF2B369E037DA9DB9493A70" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumNumberOfProductCandidatesForWhichSubstantiveMilestonePaymentsMayBeReceivedInJapan" xlink:label="loc_regn_MaximumNumberOfProductCandidatesForWhichSubstantiveMilestonePaymentsMayBeReceivedInJapan_6D7E05F30452BCD190CCDA9DB9495193" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MaximumNumberOfProductCandidatesForWhichSubstantiveMilestonePaymentsMayBeReceivedInJapan_6D7E05F30452BCD190CCDA9DB9495193" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PercentageOfDevelopmentExpensesToBeReimbursedDueToProfitabilityOfCollaborationAgreement" xlink:label="loc_regn_PercentageOfDevelopmentExpensesToBeReimbursedDueToProfitabilityOfCollaborationAgreement_2983F548919B8325C9A3DA9DB949117A" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_PercentageOfDevelopmentExpensesToBeReimbursedDueToProfitabilityOfCollaborationAgreement_2983F548919B8325C9A3DA9DB949117A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DevelopmentExpensesIncurredToDate" xlink:label="loc_regn_DevelopmentExpensesIncurredToDate_60706BA4BC942361E261DA9DB949E328" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_DevelopmentExpensesIncurredToDate_60706BA4BC942361E261DA9DB949E328" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MinimumAdvanceNoticeRequiredToTerminateCollaborationAgreement" xlink:label="loc_regn_MinimumAdvanceNoticeRequiredToTerminateCollaborationAgreement_B1AFB2C105EB653B7F88DA9DB949AAF8" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MinimumAdvanceNoticeRequiredToTerminateCollaborationAgreement_B1AFB2C105EB653B7F88DA9DB949AAF8" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NetProfitLossFromCommercializationOfProductsUnderCollaborationAgreement" xlink:label="loc_regn_NetProfitLossFromCommercializationOfProductsUnderCollaborationAgreement_3CB6ED5A626BD9B0B02FDA9DB949926C" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_NetProfitLossFromCommercializationOfProductsUnderCollaborationAgreement_3CB6ED5A626BD9B0B02FDA9DB949926C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized" xlink:label="loc_us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized_D44DBE2C7276EE123C43DA9DB949345F" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized_D44DBE2C7276EE123C43DA9DB949345F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCompensationEarned" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCompensationEarned_F6CC95743DD5BD205E5EDA9DB949D56B" xlink:type="locator" />
    <link:definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCompensationEarned_F6CC95743DD5BD205E5EDA9DB949D56B" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueRecognized" xlink:label="loc_regn_DeferredRevenueRecognized_9998E7609AC3DCEB5211DA9DB949BB24" xlink:type="locator" />
    <link:definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_DeferredRevenueRecognized_9998E7609AC3DCEB5211DA9DB949BB24" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RevenueFromRelatedParties" xlink:label="loc_us-gaap_RevenueFromRelatedParties_D83FCB4962B5BE0831AADA9DB94993AF" xlink:type="locator" />
    <link:definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_us-gaap_RevenueFromRelatedParties_D83FCB4962B5BE0831AADA9DB94993AF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DueFromRelatedPartiesCurrent" xlink:label="loc_us-gaap_DueFromRelatedPartiesCurrent_C9B05CA55937D3B6C6A7DA9DB949F173" xlink:type="locator" />
    <link:definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_us-gaap_DueFromRelatedPartiesCurrent_C9B05CA55937D3B6C6A7DA9DB949F173" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DueToRelatedPartiesCurrent" xlink:label="loc_us-gaap_DueToRelatedPartiesCurrent_3BC6E5C62C285712CDD2DA9DB9495A65" xlink:type="locator" />
    <link:definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_us-gaap_DueToRelatedPartiesCurrent_3BC6E5C62C285712CDD2DA9DB9495A65" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenue" xlink:label="loc_us-gaap_DeferredRevenue_2FC94BE17361207DA90BDA9DB9496A70" xlink:type="locator" />
    <link:definitionArc order="22" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_us-gaap_DeferredRevenue_2FC94BE17361207DA90BDA9DB9496A70" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_OtherCollaborationRevenue" xlink:label="loc_regn_OtherCollaborationRevenue_2501FD3C2F6EBF748D08DA9DB94F4C10" xlink:type="locator" />
    <link:definitionArc order="23" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_OtherCollaborationRevenue_2501FD3C2F6EBF748D08DA9DB94F4C10" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AnnualFundingMaximumOfResearchActivitiesPerAmendedAgreement" xlink:label="loc_regn_AnnualFundingMaximumOfResearchActivitiesPerAmendedAgreement_FC0EA3E9DBE552765C8BDA9DB94F0CAB" xlink:type="locator" />
    <link:definitionArc order="24" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AnnualFundingMaximumOfResearchActivitiesPerAmendedAgreement_FC0EA3E9DBE552765C8BDA9DB94F0CAB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AdjustedMaximumAmountOfAnnualFundingPerAmendedAgreement" xlink:label="loc_regn_AdjustedMaximumAmountOfAnnualFundingPerAmendedAgreement_8663BDA5FE49499704ECDA9DB94FDA93" xlink:type="locator" />
    <link:definitionArc order="25" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AdjustedMaximumAmountOfAnnualFundingPerAmendedAgreement_8663BDA5FE49499704ECDA9DB94FDA93" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AmountOfResearchFundedDuringPeriod" xlink:label="loc_regn_AmountOfResearchFundedDuringPeriod_F4370C34E8C2FA36FE7CDA9DB94F56F2" xlink:type="locator" />
    <link:definitionArc order="26" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AmountOfResearchFundedDuringPeriod_F4370C34E8C2FA36FE7CDA9DB94F56F2" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_BalanceBetweenAmountFundedAndAnnualFundingMaximum" xlink:label="loc_regn_BalanceBetweenAmountFundedAndAnnualFundingMaximum_788313E9D9095BC04A66DA9DB94FA008" xlink:type="locator" />
    <link:definitionArc order="27" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_BalanceBetweenAmountFundedAndAnnualFundingMaximum_788313E9D9095BC04A66DA9DB94FA008" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_SubstantiveMilestonePaymentToCompany" xlink:label="loc_regn_SubstantiveMilestonePaymentToCompany_86C1592D7E0257698585DA9DB94FBEC0" xlink:type="locator" />
    <link:definitionArc order="28" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_SubstantiveMilestonePaymentToCompany_86C1592D7E0257698585DA9DB94FBEC0" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NumberOfTherapeuticAntibodiesCurrentlyUnderDevelopment" xlink:label="loc_regn_NumberOfTherapeuticAntibodiesCurrentlyUnderDevelopment_B5E7D9FB4F180DB1423EDA9DB94FE0C2" xlink:type="locator" />
    <link:definitionArc order="29" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_NumberOfTherapeuticAntibodiesCurrentlyUnderDevelopment_B5E7D9FB4F180DB1423EDA9DB94FE0C2" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PercentageOfTrialCostsBorneByCollaboratingParty" xlink:label="loc_regn_PercentageOfTrialCostsBorneByCollaboratingParty_E50F41F6595E41A0C03ADA9DB94F84C4" xlink:type="locator" />
    <link:definitionArc order="30" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_PercentageOfTrialCostsBorneByCollaboratingParty_E50F41F6595E41A0C03ADA9DB94F84C4" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PercentageOfTrialCostsBorneByEntity" xlink:label="loc_regn_PercentageOfTrialCostsBorneByEntity_5690440A095A4D30F769DA9DB94F19B2" xlink:type="locator" />
    <link:definitionArc order="31" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_PercentageOfTrialCostsBorneByEntity_5690440A095A4D30F769DA9DB94F19B2" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PercentageOfTrialCostsToBeReimbursedDueToProfitabilityOfAgreement" xlink:label="loc_regn_PercentageOfTrialCostsToBeReimbursedDueToProfitabilityOfAgreement_85ADF16F3793DAC6B1D5DA9DB94F6088" xlink:type="locator" />
    <link:definitionArc order="32" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_PercentageOfTrialCostsToBeReimbursedDueToProfitabilityOfAgreement_85ADF16F3793DAC6B1D5DA9DB94F6088" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ExcessShareOfProfitNotRequiredToToBeAppliedToReimburseDevelopmentCost" xlink:label="loc_regn_ExcessShareOfProfitNotRequiredToToBeAppliedToReimburseDevelopmentCost_680E977E33A79F1A14F4DA9DB94F71DB" xlink:type="locator" />
    <link:definitionArc order="33" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_ExcessShareOfProfitNotRequiredToToBeAppliedToReimburseDevelopmentCost_680E977E33A79F1A14F4DA9DB94F71DB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_StartingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCollaboratingParty" xlink:label="loc_regn_StartingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCollaboratingParty_4E7D08532BDC5E0D3BF2DA9DB94FAA85" xlink:type="locator" />
    <link:definitionArc order="34" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_StartingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCollaboratingParty_4E7D08532BDC5E0D3BF2DA9DB94FAA85" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_StartingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCompany" xlink:label="loc_regn_StartingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCompany_355FFEC8435AB0CCAB2EDA9DB94F8543" xlink:type="locator" />
    <link:definitionArc order="35" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_StartingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCompany_355FFEC8435AB0CCAB2EDA9DB94F8543" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_EndingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCollaboratingParty" xlink:label="loc_regn_EndingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCollaboratingParty_7926B71DD2149EAE9579DA9DB94F5BF5" xlink:type="locator" />
    <link:definitionArc order="36" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_EndingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCollaboratingParty_7926B71DD2149EAE9579DA9DB94F5BF5" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_EndingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCompany" xlink:label="loc_regn_EndingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCompany_38BA97F5C990454EE678DA9DB94F320B" xlink:type="locator" />
    <link:definitionArc order="37" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_EndingShareOfProfitsOutsideUnitedStatesBasedOnSalesForCompany_38BA97F5C990454EE678DA9DB94F320B" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ShareOfLossesOutsideUnitedStatesForCollaboratingParty" xlink:label="loc_regn_ShareOfLossesOutsideUnitedStatesForCollaboratingParty_5D47A2626B8488DA6DFADA9DB9508973" xlink:type="locator" />
    <link:definitionArc order="38" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_ShareOfLossesOutsideUnitedStatesForCollaboratingParty_5D47A2626B8488DA6DFADA9DB9508973" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ShareOfLossesOutsideUnitedStatesForCompany" xlink:label="loc_regn_ShareOfLossesOutsideUnitedStatesForCompany_13FA28AEDA8FFC2BE640DA9DB95081B3" xlink:type="locator" />
    <link:definitionArc order="39" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_ShareOfLossesOutsideUnitedStatesForCompany_13FA28AEDA8FFC2BE640DA9DB95081B3" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumAmountOfMilestonePaymentsThatMayBeReceivedIfCertainSalesAreAchieved" xlink:label="loc_regn_MaximumAmountOfMilestonePaymentsThatMayBeReceivedIfCertainSalesAreAchieved_3674CB23AA990EAD8DB4DA9DB950162A" xlink:type="locator" />
    <link:definitionArc order="40" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MaximumAmountOfMilestonePaymentsThatMayBeReceivedIfCertainSalesAreAchieved_3674CB23AA990EAD8DB4DA9DB950162A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AmountOfFundingOfAgreedUponCostIncurredToExpandManufacturingCapacity" xlink:label="loc_regn_AmountOfFundingOfAgreedUponCostIncurredToExpandManufacturingCapacity_FE6D83D81314F7EE3975DA9DB9504773" xlink:type="locator" />
    <link:definitionArc order="41" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AmountOfFundingOfAgreedUponCostIncurredToExpandManufacturingCapacity_FE6D83D81314F7EE3975DA9DB9504773" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PeriodOfNoticeToOptOutOfFurtherDevelopmentAndOrCommercialization" xlink:label="loc_regn_PeriodOfNoticeToOptOutOfFurtherDevelopmentAndOrCommercialization_56147EDDDC1652D86B86DA9DB9503F0C" xlink:type="locator" />
    <link:definitionArc order="42" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_PeriodOfNoticeToOptOutOfFurtherDevelopmentAndOrCommercialization_56147EDDDC1652D86B86DA9DB9503F0C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PeriodOfNoticeToOptOutOfFurtherDevelopmentUponEnteringOfJointDevelopmentProduct" xlink:label="loc_regn_PeriodOfNoticeToOptOutOfFurtherDevelopmentUponEnteringOfJointDevelopmentProduct_52E06A959FA7E52AD3CEDA9DB9500211" xlink:type="locator" />
    <link:definitionArc order="43" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_PeriodOfNoticeToOptOutOfFurtherDevelopmentUponEnteringOfJointDevelopmentProduct_52E06A959FA7E52AD3CEDA9DB9500211" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MinimumPaymentsExpectedToBeReceivedUnderAgreement" xlink:label="loc_regn_MinimumPaymentsExpectedToBeReceivedUnderAgreement_BCA087372A788A68208FDA9DB950C50C" xlink:type="locator" />
    <link:definitionArc order="44" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MinimumPaymentsExpectedToBeReceivedUnderAgreement_BCA087372A788A68208FDA9DB950C50C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AmountOfMilestonePaymentsEarned" xlink:label="loc_regn_AmountOfMilestonePaymentsEarned_11DEE818EAEA4BF7F089DA9DB950EF5D" xlink:type="locator" />
    <link:definitionArc order="45" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AmountOfMilestonePaymentsEarned_11DEE818EAEA4BF7F089DA9DB950EF5D" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInMajorMarketCountriesOutsideUS" xlink:label="loc_regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInMajorMarketCountriesOutsideUS_8598DD825A090644C937DA9DB9500604" xlink:type="locator" />
    <link:definitionArc order="46" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MaximumAmountOfSubstantiveMilestonePaymentsThatMayBeReceivedInMajorMarketCountriesOutsideUS_8598DD825A090644C937DA9DB9500604" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumAmountOfSalesMilestonePaymentsIfTotalSalesAchieveSpecificLevels" xlink:label="loc_regn_MaximumAmountOfSalesMilestonePaymentsIfTotalSalesAchieveSpecificLevels_D6C03ECCBDCB4938A4F8DA9DB95013E5" xlink:type="locator" />
    <link:definitionArc order="47" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MaximumAmountOfSalesMilestonePaymentsIfTotalSalesAchieveSpecificLevels_D6C03ECCBDCB4938A4F8DA9DB95013E5" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_StartingSpecifiedLevelsOfAnnualSalesAtWhichSalesMilestonePaymentsMayBeReceived" xlink:label="loc_regn_StartingSpecifiedLevelsOfAnnualSalesAtWhichSalesMilestonePaymentsMayBeReceived_25EE852CB7CDC4DF041BDA9DB9503327" xlink:type="locator" />
    <link:definitionArc order="48" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_StartingSpecifiedLevelsOfAnnualSalesAtWhichSalesMilestonePaymentsMayBeReceived_25EE852CB7CDC4DF041BDA9DB9503327" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PeriodForAchievingSalesTargetForMilestonePaymentRollingBasis" xlink:label="loc_regn_PeriodForAchievingSalesTargetForMilestonePaymentRollingBasis_85C6969AADC67F63E5A4DA9DB950D3FF" xlink:type="locator" />
    <link:definitionArc order="49" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_PeriodForAchievingSalesTargetForMilestonePaymentRollingBasis_85C6969AADC67F63E5A4DA9DB950D3FF" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AdditionalResearchAndDevelopmentExpensesObligatedToReimburse" xlink:label="loc_regn_AdditionalResearchAndDevelopmentExpensesObligatedToReimburse_71D472DDA2D3BAC379DDDA9DB950765D" xlink:type="locator" />
    <link:definitionArc order="50" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AdditionalResearchAndDevelopmentExpensesObligatedToReimburse_71D472DDA2D3BAC379DDDA9DB950765D" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AccountReceivableFromBayerHealthcare" xlink:label="loc_regn_AccountReceivableFromBayerHealthcare_000F9B30DD4E80F83A0FDA9DB9508103" xlink:type="locator" />
    <link:definitionArc order="51" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AccountReceivableFromBayerHealthcare_000F9B30DD4E80F83A0FDA9DB9508103" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MinimumAmountOfResearchFunding" xlink:label="loc_regn_MinimumAmountOfResearchFunding_A49A76A0176F4FD1D8FEDA9DB9514045" xlink:type="locator" />
    <link:definitionArc order="52" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_MinimumAmountOfResearchFunding_A49A76A0176F4FD1D8FEDA9DB9514045" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PeriodOfFunding" xlink:label="loc_regn_PeriodOfFunding_0043E00AFFEC4BCC5B35DA9DB951580E" xlink:type="locator" />
    <link:definitionArc order="53" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_PeriodOfFunding_0043E00AFFEC4BCC5B35DA9DB951580E" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AdditionalFundingToOptimizeCertainExistingTechnology" xlink:label="loc_regn_AdditionalFundingToOptimizeCertainExistingTechnology_9118A9BBE379ABAFBD4FDA9DB951E175" xlink:type="locator" />
    <link:definitionArc order="54" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_regn_AdditionalFundingToOptimizeCertainExistingTechnology_9118A9BBE379ABAFBD4FDA9DB951E175" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ContractsRevenue" xlink:label="loc_us-gaap_ContractsRevenue_185FD7CD19CDA128B0C7DA9DB9519F82" xlink:type="locator" />
    <link:definitionArc order="55" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_B5FBB0985247F1581F18DA9DB948FEB2" xlink:to="loc_us-gaap_ContractsRevenue_185FD7CD19CDA128B0C7DA9DB9519F82" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/CommitmentsAndContingenciesCapitalLeasesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CapitalLeasedAssetsLineItems" xlink:label="loc_us-gaap_CapitalLeasedAssetsLineItems_F1370251564141BD7EE5CABBCD9E70A1" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfCapitalLeasedAsssetsTable" xlink:label="loc_us-gaap_ScheduleOfCapitalLeasedAsssetsTable_017088DF0DA9CDDBDFB7CA5093E02D97" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CapitalLeasedAssetsLineItems_F1370251564141BD7EE5CABBCD9E70A1" xlink:to="loc_us-gaap_ScheduleOfCapitalLeasedAsssetsTable_017088DF0DA9CDDBDFB7CA5093E02D97" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseAxis" xlink:label="loc_regn_NameOfLeaseAxis_D55C0059F59475964A92CA5093E02D5F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCapitalLeasedAsssetsTable_017088DF0DA9CDDBDFB7CA5093E02D97" xlink:to="loc_regn_NameOfLeaseAxis_D55C0059F59475964A92CA5093E02D5F" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseDomain" xlink:label="loc_regn_NameOfLeaseDomain_F1A879F59F119834D3B7CA5093E0AA3A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_NameOfLeaseAxis_D55C0059F59475964A92CA5093E02D5F" xlink:to="loc_regn_NameOfLeaseDomain_F1A879F59F119834D3B7CA5093E0AA3A_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseDomain" xlink:label="loc_regn_NameOfLeaseDomain_F1A879F59F119834D3B7CA5093E0AA3A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_NameOfLeaseAxis_D55C0059F59475964A92CA5093E02D5F" xlink:to="loc_regn_NameOfLeaseDomain_F1A879F59F119834D3B7CA5093E0AA3A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_TarrytownLeaseMember" xlink:label="loc_regn_TarrytownLeaseMember_D1C2556F0396405A34DDCA5093E0080A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_NameOfLeaseAxis_D55C0059F59475964A92CA5093E02D5F" xlink:to="loc_regn_TarrytownLeaseMember_D1C2556F0396405A34DDCA5093E0080A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_0FB09E8209F18A2264F4CA5093E00F9C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCapitalLeasedAsssetsTable_017088DF0DA9CDDBDFB7CA5093E02D97" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_0FB09E8209F18A2264F4CA5093E00F9C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_771C2D9F6CC41E3C077ACA5093E0CED5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_0FB09E8209F18A2264F4CA5093E00F9C" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_771C2D9F6CC41E3C077ACA5093E0CED5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_771C2D9F6CC41E3C077ACA5093E0CED5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_0FB09E8209F18A2264F4CA5093E00F9C" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_771C2D9F6CC41E3C077ACA5093E0CED5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EquipmentMember" xlink:label="loc_us-gaap_EquipmentMember_951B592162B97E4DFFE0CA5093E09EA1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_771C2D9F6CC41E3C077ACA5093E0CED5" xlink:to="loc_us-gaap_EquipmentMember_951B592162B97E4DFFE0CA5093E09EA1" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MarketableDebtSecuritiesCollateral" xlink:label="loc_regn_MarketableDebtSecuritiesCollateral_17878324D5AF7761A080CAB5B4794A7E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasedAssetsLineItems_F1370251564141BD7EE5CABBCD9E70A1" xlink:to="loc_regn_MarketableDebtSecuritiesCollateral_17878324D5AF7761A080CAB5B4794A7E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract" xlink:label="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_E2E8E28ACEE89B3AC260CAA5CDC2EB2B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasedAssetsLineItems_F1370251564141BD7EE5CABBCD9E70A1" xlink:to="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_E2E8E28ACEE89B3AC260CAA5CDC2EB2B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent" xlink:label="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent_06533297432CB720B523CAA5CDC2AC20" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_E2E8E28ACEE89B3AC260CAA5CDC2EB2B" xlink:to="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent_06533297432CB720B523CAA5CDC2AC20" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears" xlink:label="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears_BFD86F2DDCD3C92E4834CAA5CDC2770E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_E2E8E28ACEE89B3AC260CAA5CDC2EB2B" xlink:to="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears_BFD86F2DDCD3C92E4834CAA5CDC2770E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CapitalLeasesFutureMinimumPaymentsDue" xlink:label="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDue_06203D5A7A1FFF1DA7B8CAA5CDC21F52" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_E2E8E28ACEE89B3AC260CAA5CDC2EB2B" xlink:to="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDue_06203D5A7A1FFF1DA7B8CAA5CDC21F52" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CapitalLeasesFutureMinimumPaymentsInterestIncludedInPayments" xlink:label="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsInterestIncludedInPayments_4C044DEF7E30E6AB711BCAA5CDC2F949" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_E2E8E28ACEE89B3AC260CAA5CDC2EB2B" xlink:to="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsInterestIncludedInPayments_4C044DEF7E30E6AB711BCAA5CDC2F949" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CapitalLeasesFutureMinimumPaymentsPresentValueOfNetMinimumPayments" xlink:label="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsPresentValueOfNetMinimumPayments_03F2D69DFC4AA41510FFCAA5CDC28288" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_E2E8E28ACEE89B3AC260CAA5CDC2EB2B" xlink:to="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsPresentValueOfNetMinimumPayments_03F2D69DFC4AA41510FFCAA5CDC28288" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CapitalLeaseObligationsNoncurrent" xlink:label="loc_us-gaap_CapitalLeaseObligationsNoncurrent_B271EA6B03CD41788F4BCAA5CDC27A7B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_E2E8E28ACEE89B3AC260CAA5CDC2EB2B" xlink:to="loc_us-gaap_CapitalLeaseObligationsNoncurrent_B271EA6B03CD41788F4BCAA5CDC27A7B" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/CommitmentsAndContingenciesFacilityLeasesDetails" xlink:type="extended">
    <link:loc xlink:href="regn-20121231.xsd#regn_ScheduleOfFacilityLeasesLineItems" xlink:label="loc_regn_ScheduleOfFacilityLeasesLineItems_455BE34A68D3754ACA35CAB9971B6744" xlink:type="locator" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ScheduleOfFacilityLeasesTable" xlink:label="loc_regn_ScheduleOfFacilityLeasesTable_8F68F6BDE97527825476CA5093E09CF5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_regn_ScheduleOfFacilityLeasesLineItems_455BE34A68D3754ACA35CAB9971B6744" xlink:to="loc_regn_ScheduleOfFacilityLeasesTable_8F68F6BDE97527825476CA5093E09CF5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseAxis" xlink:label="loc_regn_NameOfLeaseAxis_53282C47238DD128DB03CA5093E08EF9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_regn_ScheduleOfFacilityLeasesTable_8F68F6BDE97527825476CA5093E09CF5" xlink:to="loc_regn_NameOfLeaseAxis_53282C47238DD128DB03CA5093E08EF9" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseDomain" xlink:label="loc_regn_NameOfLeaseDomain_E856A12F833BB46C0F99CA5093E068B8_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_NameOfLeaseAxis_53282C47238DD128DB03CA5093E08EF9" xlink:to="loc_regn_NameOfLeaseDomain_E856A12F833BB46C0F99CA5093E068B8_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseDomain" xlink:label="loc_regn_NameOfLeaseDomain_E856A12F833BB46C0F99CA5093E068B8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_NameOfLeaseAxis_53282C47238DD128DB03CA5093E08EF9" xlink:to="loc_regn_NameOfLeaseDomain_E856A12F833BB46C0F99CA5093E068B8" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_TarrytownLeaseMember" xlink:label="loc_regn_TarrytownLeaseMember_34BD0C43187E63658260CA5093E0CBC4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_NameOfLeaseAxis_53282C47238DD128DB03CA5093E08EF9" xlink:to="loc_regn_TarrytownLeaseMember_34BD0C43187E63658260CA5093E0CBC4" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeaseObligationsAxis" xlink:label="loc_regn_FacilityLeaseObligationsAxis_1BEEC0C7D0339C849F2FCA5093E0127B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_regn_ScheduleOfFacilityLeasesTable_8F68F6BDE97527825476CA5093E09CF5" xlink:to="loc_regn_FacilityLeaseObligationsAxis_1BEEC0C7D0339C849F2FCA5093E0127B" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeaseObligationsNameDomain" xlink:label="loc_regn_FacilityLeaseObligationsNameDomain_6A45AC09E1138CC122DFCA5093E0B340_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_FacilityLeaseObligationsAxis_1BEEC0C7D0339C849F2FCA5093E0127B" xlink:to="loc_regn_FacilityLeaseObligationsNameDomain_6A45AC09E1138CC122DFCA5093E0B340_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeaseObligationsNameDomain" xlink:label="loc_regn_FacilityLeaseObligationsNameDomain_6A45AC09E1138CC122DFCA5093E0B340" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_FacilityLeaseObligationsAxis_1BEEC0C7D0339C849F2FCA5093E0127B" xlink:to="loc_regn_FacilityLeaseObligationsNameDomain_6A45AC09E1138CC122DFCA5093E0B340" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilitiesBuildingsAndBMember" xlink:label="loc_regn_FacilitiesBuildingsAndBMember_1227A62D4FF0A976D3B3CA5093E02353" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeaseObligationsNameDomain_6A45AC09E1138CC122DFCA5093E0B340" xlink:to="loc_regn_FacilitiesBuildingsAndBMember_1227A62D4FF0A976D3B3CA5093E02353" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilitiesBuildingCMember" xlink:label="loc_regn_FacilitiesBuildingCMember_B0F6E58925F84D59A3E7CA5093E0373F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeaseObligationsNameDomain_6A45AC09E1138CC122DFCA5093E0B340" xlink:to="loc_regn_FacilitiesBuildingCMember_B0F6E58925F84D59A3E7CA5093E0373F" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeaseObligationsAbstract" xlink:label="loc_regn_FacilityLeaseObligationsAbstract_39B3CF77AA936ED08168CAA39F1341B9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ScheduleOfFacilityLeasesLineItems_455BE34A68D3754ACA35CAB9971B6744" xlink:to="loc_regn_FacilityLeaseObligationsAbstract_39B3CF77AA936ED08168CAA39F1341B9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_BuildingsAndImprovementsGross" xlink:label="loc_us-gaap_BuildingsAndImprovementsGross_6EBAB06F07BFA48FA105CAA39F13BE88" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeaseObligationsAbstract_39B3CF77AA936ED08168CAA39F1341B9" xlink:to="loc_us-gaap_BuildingsAndImprovementsGross_6EBAB06F07BFA48FA105CAA39F13BE88" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ReimbursementsRecognizedAsAdditionalFacilityLeaseObligation" xlink:label="loc_regn_ReimbursementsRecognizedAsAdditionalFacilityLeaseObligation_41AF2CE598B1B180C3C4CAA39F137B5E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeaseObligationsAbstract_39B3CF77AA936ED08168CAA39F1341B9" xlink:to="loc_regn_ReimbursementsRecognizedAsAdditionalFacilityLeaseObligation_41AF2CE598B1B180C3C4CAA39F137B5E" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ImputedInterestRateApplicableToFacilityLeaseObligation" xlink:label="loc_regn_ImputedInterestRateApplicableToFacilityLeaseObligation_1CECB2866A1B461AB10ACAA39F130C53" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeaseObligationsAbstract_39B3CF77AA936ED08168CAA39F1341B9" xlink:to="loc_regn_ImputedInterestRateApplicableToFacilityLeaseObligation_1CECB2866A1B461AB10ACAA39F130C53" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_InterestExpenseInConnectionWithFacilityLeaseObligation" xlink:label="loc_regn_InterestExpenseInConnectionWithFacilityLeaseObligation_B6597F4441057B6EC8E5CAA39F13EDFA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeaseObligationsAbstract_39B3CF77AA936ED08168CAA39F1341B9" xlink:to="loc_regn_InterestExpenseInConnectionWithFacilityLeaseObligation_B6597F4441057B6EC8E5CAA39F13EDFA" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeaseObligationIncludingReimbursements" xlink:label="loc_regn_FacilityLeaseObligationIncludingReimbursements_14582905A08A6FC79572CAA39F132C30" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeaseObligationsAbstract_39B3CF77AA936ED08168CAA39F1341B9" xlink:to="loc_regn_FacilityLeaseObligationIncludingReimbursements_14582905A08A6FC79572CAA39F132C30" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesFutureMinimumPaymentsDueAbstract" xlink:label="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ScheduleOfFacilityLeasesLineItems_455BE34A68D3754ACA35CAB9971B6744" xlink:to="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesFutureMinimumPaymentsDueCurrent" xlink:label="loc_regn_FacilityLeasesFutureMinimumPaymentsDueCurrent_7ABFE1EFBC71826A4BFFCAA39F1D883A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:to="loc_regn_FacilityLeasesFutureMinimumPaymentsDueCurrent_7ABFE1EFBC71826A4BFFCAA39F1D883A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesFutureMinimumPaymentsDueDueInTwoYears" xlink:label="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueInTwoYears_0E841DF4B2620E365364CAA39F1DF78C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:to="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueInTwoYears_0E841DF4B2620E365364CAA39F1DF78C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesFutureMinimumPaymentsDueDueInThreeYears" xlink:label="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueInThreeYears_72E96E4651FB319482EFCAA39F1DABCA" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:to="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueInThreeYears_72E96E4651FB319482EFCAA39F1DABCA" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesFutureMinimumPaymentsDueDueInFourYears" xlink:label="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueInFourYears_D3F54572034E608113B7CAA39F1D4904" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:to="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueInFourYears_D3F54572034E608113B7CAA39F1D4904" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesFutureMinimumPaymentsDueDueInFiveYears" xlink:label="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueInFiveYears_A214189AFACF94E2B20ECAA39F1DE2BC" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:to="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueInFiveYears_A214189AFACF94E2B20ECAA39F1DE2BC" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesFutureMinimumPaymentsDueDueThereafter" xlink:label="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueThereafter_FA10ED36C484C17F785FCAA39F1D3C79" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:to="loc_regn_FacilityLeasesFutureMinimumPaymentsDueDueThereafter_FA10ED36C484C17F785FCAA39F1D3C79" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesFutureMinimumPaymentsDue" xlink:label="loc_regn_FacilityLeasesFutureMinimumPaymentsDue_83AB340302185D6DE898CAA39F1D0B2A" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FacilityLeasesFutureMinimumPaymentsDueAbstract_4430324F8BBFD7A03ECECAA39F1DC287" xlink:to="loc_regn_FacilityLeasesFutureMinimumPaymentsDue_83AB340302185D6DE898CAA39F1D0B2A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/CommitmentsAndContingenciesOperatingLeasesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasedAssetsLineItems" xlink:label="loc_us-gaap_OperatingLeasedAssetsLineItems_164A626F9B84E7C20490CFEC01BFF44F" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfOperatingLeasedAssetsTable" xlink:label="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_269EC644A5CE284CEF54CFEC01BE522E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_164A626F9B84E7C20490CFEC01BFF44F" xlink:to="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_269EC644A5CE284CEF54CFEC01BE522E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseAxis" xlink:label="loc_regn_NameOfLeaseAxis_44549B580935B7DEC78DCFEC01BEB6C0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_269EC644A5CE284CEF54CFEC01BE522E" xlink:to="loc_regn_NameOfLeaseAxis_44549B580935B7DEC78DCFEC01BEB6C0" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseDomain" xlink:label="loc_regn_NameOfLeaseDomain_16FF260EF72AFF35F31CCFEC01BE6F07_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_NameOfLeaseAxis_44549B580935B7DEC78DCFEC01BEB6C0" xlink:to="loc_regn_NameOfLeaseDomain_16FF260EF72AFF35F31CCFEC01BE6F07_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NameOfLeaseDomain" xlink:label="loc_regn_NameOfLeaseDomain_16FF260EF72AFF35F31CCFEC01BE6F07" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_NameOfLeaseAxis_44549B580935B7DEC78DCFEC01BEB6C0" xlink:to="loc_regn_NameOfLeaseDomain_16FF260EF72AFF35F31CCFEC01BE6F07" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_TarrytownLeaseMember" xlink:label="loc_regn_TarrytownLeaseMember_587D5E0E837375C707B7CFEC01BE2170" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_NameOfLeaseDomain_16FF260EF72AFF35F31CCFEC01BE6F07" xlink:to="loc_regn_TarrytownLeaseMember_587D5E0E837375C707B7CFEC01BE2170" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1D5680723EDDC8B4D984CFEC01BFA100" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_269EC644A5CE284CEF54CFEC01BE522E" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1D5680723EDDC8B4D984CFEC01BFA100" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43521E210E2BE2BDCFF5CFEC01BFCA44_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1D5680723EDDC8B4D984CFEC01BFA100" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43521E210E2BE2BDCFF5CFEC01BFCA44_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43521E210E2BE2BDCFF5CFEC01BFCA44" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_1D5680723EDDC8B4D984CFEC01BFA100" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43521E210E2BE2BDCFF5CFEC01BFCA44" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_BuildingMember" xlink:label="loc_us-gaap_BuildingMember_C91B1212E71DFD6E08CBCFEC01BF5EDF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43521E210E2BE2BDCFF5CFEC01BFCA44" xlink:to="loc_us-gaap_BuildingMember_C91B1212E71DFD6E08CBCFEC01BF5EDF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EquipmentMember" xlink:label="loc_us-gaap_EquipmentMember_8553F280656B879C4BC0CFEC01BF2339" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43521E210E2BE2BDCFF5CFEC01BFCA44" xlink:to="loc_us-gaap_EquipmentMember_8553F280656B879C4BC0CFEC01BF2339" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LeasesOperatingAbstract" xlink:label="loc_us-gaap_LeasesOperatingAbstract_7E3D6AD9265322C1DC79CFEC01BF1BEF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_164A626F9B84E7C20490CFEC01BFF44F" xlink:to="loc_us-gaap_LeasesOperatingAbstract_7E3D6AD9265322C1DC79CFEC01BF1BEF" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NumberOfNewBuildingsLeased" xlink:label="loc_regn_NumberOfNewBuildingsLeased_1A1B9EEED9040CFB6A2ACFEC01BF383F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeasesOperatingAbstract_7E3D6AD9265322C1DC79CFEC01BF1BEF" xlink:to="loc_regn_NumberOfNewBuildingsLeased_1A1B9EEED9040CFB6A2ACFEC01BF383F" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesNumberOfRenewalOptions" xlink:label="loc_regn_FacilityLeasesNumberOfRenewalOptions_B1BAB31FEC01F3ED34A8D01202A95556" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeasesOperatingAbstract_7E3D6AD9265322C1DC79CFEC01BF1BEF" xlink:to="loc_regn_FacilityLeasesNumberOfRenewalOptions_B1BAB31FEC01F3ED34A8D01202A95556" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesLeaseExtensionTerm" xlink:label="loc_regn_FacilityLeasesLeaseExtensionTerm_3B67E7EBC1AA583B06E2D012E7CFE988" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeasesOperatingAbstract_7E3D6AD9265322C1DC79CFEC01BF1BEF" xlink:to="loc_regn_FacilityLeasesLeaseExtensionTerm_3B67E7EBC1AA583B06E2D012E7CFE988" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeasesLeaseRenewalEscalationPercentage" xlink:label="loc_regn_FacilityLeasesLeaseRenewalEscalationPercentage_2A5C83CFD756C82C7194D0140BAC4DBE" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeasesOperatingAbstract_7E3D6AD9265322C1DC79CFEC01BF1BEF" xlink:to="loc_regn_FacilityLeasesLeaseRenewalEscalationPercentage_2A5C83CFD756C82C7194D0140BAC4DBE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_B91524E7508192B6984ECFEC01BFF357" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeasesOperatingAbstract_7E3D6AD9265322C1DC79CFEC01BF1BEF" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_B91524E7508192B6984ECFEC01BFF357" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CashAndMarketableDebtSecuritiesCollateral" xlink:label="loc_regn_CashAndMarketableDebtSecuritiesCollateral_ABB1D8ACEBD670B8D9B7CFEC01BF0572" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeasesOperatingAbstract_7E3D6AD9265322C1DC79CFEC01BF1BEF" xlink:to="loc_regn_CashAndMarketableDebtSecuritiesCollateral_ABB1D8ACEBD670B8D9B7CFEC01BF0572" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_164A626F9B84E7C20490CFEC01BFF44F" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent_8BB1A67288ED16FBB25BCFEC01BF1811" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent_8BB1A67288ED16FBB25BCFEC01BF1811" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears_30DEF5609D13557D4D69CFEC01BF361E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears_30DEF5609D13557D4D69CFEC01BF361E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_89695A4191161C3E8AA6CFEC01BFC9CF" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_89695A4191161C3E8AA6CFEC01BFC9CF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_953E9F36B8EBB2E86B3ECFEC01BFF3DD" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_953E9F36B8EBB2E86B3ECFEC01BFF3DD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears_583C3048FC34F029619CCFEC01BFFAB9" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears_583C3048FC34F029619CCFEC01BFFAB9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter_0B3477EC8B8A3C718EAFCFEC01BFF629" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter_0B3477EC8B8A3C718EAFCFEC01BFF629" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_1A7EF89FFD7CFA52CDE3CFEC01C01DDC" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_9D3291FF5EC236E4A105CFEC01BF1E74" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_1A7EF89FFD7CFA52CDE3CFEC01C01DDC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesRentExpenseAbstract" xlink:label="loc_us-gaap_OperatingLeasesRentExpenseAbstract_5DCF26225A32F9FC446CCFEC01C0394E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_164A626F9B84E7C20490CFEC01BFF44F" xlink:to="loc_us-gaap_OperatingLeasesRentExpenseAbstract_5DCF26225A32F9FC446CCFEC01C0394E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasesRentExpenseNet" xlink:label="loc_us-gaap_OperatingLeasesRentExpenseNet_BCDD128424B02AB9288BCFEC01C0674A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesRentExpenseAbstract_5DCF26225A32F9FC446CCFEC01C0394E" xlink:to="loc_us-gaap_OperatingLeasesRentExpenseNet_BCDD128424B02AB9288BCFEC01C0674A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RentChargesForUtilitiesRealEstateTaxesAndOperatingExpense" xlink:label="loc_regn_RentChargesForUtilitiesRealEstateTaxesAndOperatingExpense_37A207E73F7357DF70DCCFEC01C0F55D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesRentExpenseAbstract_5DCF26225A32F9FC446CCFEC01C0394E" xlink:to="loc_regn_RentChargesForUtilitiesRealEstateTaxesAndOperatingExpense_37A207E73F7357DF70DCCFEC01C0F55D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/CommitmentsAndContingenciesResearchCollaborationAndLicensingAgreementsDetails" xlink:type="extended">
    <link:loc xlink:href="regn-20121231.xsd#regn_ResearchCollaborationAndLicensingArrangementsLineItems" xlink:label="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:type="locator" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ResearchCollaborationAndLicensingArrangementsTable" xlink:label="loc_regn_ResearchCollaborationAndLicensingArrangementsTable_8FB5406B4ECC42670B58CA5093D63E65" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_ResearchCollaborationAndLicensingArrangementsTable_8FB5406B4ECC42670B58CA5093D63E65" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ResearchCollaborationAndLicensingAgreementsAxis" xlink:label="loc_regn_ResearchCollaborationAndLicensingAgreementsAxis_50952AB794F951AD31CBCA5093D61ABE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsTable_8FB5406B4ECC42670B58CA5093D63E65" xlink:to="loc_regn_ResearchCollaborationAndLicensingAgreementsAxis_50952AB794F951AD31CBCA5093D61ABE" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ResearchCollaborationAndLicensingAgreementsDomain" xlink:label="loc_regn_ResearchCollaborationAndLicensingAgreementsDomain_CEE6429A7630250BC5C0CA5093D68BB5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_ResearchCollaborationAndLicensingAgreementsAxis_50952AB794F951AD31CBCA5093D61ABE" xlink:to="loc_regn_ResearchCollaborationAndLicensingAgreementsDomain_CEE6429A7630250BC5C0CA5093D68BB5_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ResearchCollaborationAndLicensingAgreementsDomain" xlink:label="loc_regn_ResearchCollaborationAndLicensingAgreementsDomain_CEE6429A7630250BC5C0CA5093D68BB5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_ResearchCollaborationAndLicensingAgreementsAxis_50952AB794F951AD31CBCA5093D61ABE" xlink:to="loc_regn_ResearchCollaborationAndLicensingAgreementsDomain_CEE6429A7630250BC5C0CA5093D68BB5" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AgreementsWithRoyaltyProvisionsMember" xlink:label="loc_regn_AgreementsWithRoyaltyProvisionsMember_72024612C680C08E417DCA5093D67349" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingAgreementsDomain_CEE6429A7630250BC5C0CA5093D68BB5" xlink:to="loc_regn_AgreementsWithRoyaltyProvisionsMember_72024612C680C08E417DCA5093D67349" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LicenseAgreementWithGenentechMember" xlink:label="loc_regn_LicenseAgreementWithGenentechMember_2EE9C713A2F333D92B00CA5093D6B260" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingAgreementsDomain_CEE6429A7630250BC5C0CA5093D68BB5" xlink:to="loc_regn_LicenseAgreementWithGenentechMember_2EE9C713A2F333D92B00CA5093D6B260" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CellectisAgreementMember" xlink:label="loc_regn_CellectisAgreementMember_F98ADB95875B2A9D4DBECA5093D65D5A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingAgreementsDomain_CEE6429A7630250BC5C0CA5093D68BB5" xlink:to="loc_regn_CellectisAgreementMember_F98ADB95875B2A9D4DBECA5093D65D5A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RoyaltyRateMinimum" xlink:label="loc_regn_RoyaltyRateMinimum_06426DD53369870352E6CA5093D6075D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_RoyaltyRateMinimum_06426DD53369870352E6CA5093D6075D" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RoyaltyRateMaximum" xlink:label="loc_regn_RoyaltyRateMaximum_3725122E04F7D6069286CA5093D65C44" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_RoyaltyRateMaximum_3725122E04F7D6069286CA5093D65C44" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AmountOfPaymentToBeMadeUponReachingMinimumSalesTarget" xlink:label="loc_regn_AmountOfPaymentToBeMadeUponReachingMinimumSalesTarget_098CB1F2E3395939DA9ECA5093D6C5D0" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_AmountOfPaymentToBeMadeUponReachingMinimumSalesTarget_098CB1F2E3395939DA9ECA5093D6C5D0" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CumulativeSalesRangeMinimum" xlink:label="loc_regn_CumulativeSalesRangeMinimum_6A49492CA65F86A48D96CA5093D6C8CA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_CumulativeSalesRangeMinimum_6A49492CA65F86A48D96CA5093D6C8CA" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CumulativeSalesRangeMaximum" xlink:label="loc_regn_CumulativeSalesRangeMaximum_FCD2913DF0E15B4ADCD6CA5093D61621" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_CumulativeSalesRangeMaximum_FCD2913DF0E15B4ADCD6CA5093D61621" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RoyaltyPaymentPercentageOfCumulativeSalesWithinRange" xlink:label="loc_regn_RoyaltyPaymentPercentageOfCumulativeSalesWithinRange_A467862D15AA51FBF904CA5093D62A08" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_RoyaltyPaymentPercentageOfCumulativeSalesWithinRange_A467862D15AA51FBF904CA5093D62A08" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RoyaltyPaymentPercentageOfCumulativeSalesOverRange" xlink:label="loc_regn_RoyaltyPaymentPercentageOfCumulativeSalesOverRange_1238F8F44C5933FBF5D7CA5093D6C51E" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_RoyaltyPaymentPercentageOfCumulativeSalesOverRange_1238F8F44C5933FBF5D7CA5093D6C51E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RoyaltyExpense" xlink:label="loc_us-gaap_RoyaltyExpense_D73C45DEB756DCE66BE6CA5093D6A2EA" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_us-gaap_RoyaltyExpense_D73C45DEB756DCE66BE6CA5093D6A2EA" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NonRefundableAdvancePaymentMadeUnderLicenseAgreement" xlink:label="loc_regn_NonRefundableAdvancePaymentMadeUnderLicenseAgreement_CACBE71C0A66E01C8AD2CA5093D6B0FB" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_NonRefundableAdvancePaymentMadeUnderLicenseAgreement_CACBE71C0A66E01C8AD2CA5093D6B0FB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AmortizationOfAdvancePayment" xlink:label="loc_regn_AmortizationOfAdvancePayment_4421E7DE32141612370BCA5093D64BA9" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_AmortizationOfAdvancePayment_4421E7DE32141612370BCA5093D64BA9" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_UnamortizedBalanceOfAdvancePayment" xlink:label="loc_regn_UnamortizedBalanceOfAdvancePayment_A0FEBE6F5B7043617C6DCA5093D66BDD" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_UnamortizedBalanceOfAdvancePayment_A0FEBE6F5B7043617C6DCA5093D66BDD" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FutureAmortizationExpenseOfAdvancePaymentAbstract" xlink:label="loc_regn_FutureAmortizationExpenseOfAdvancePaymentAbstract_CBF029C4AC23CADE5534CA5093D60B70" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ResearchCollaborationAndLicensingArrangementsLineItems_1AA91A311A917F78D882CA5093D61209" xlink:to="loc_regn_FutureAmortizationExpenseOfAdvancePaymentAbstract_CBF029C4AC23CADE5534CA5093D60B70" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FutureExpenseOfAdvancePaymentYearOne" xlink:label="loc_regn_FutureExpenseOfAdvancePaymentYearOne_D507C439519DC0967538CA5093D68ADB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FutureAmortizationExpenseOfAdvancePaymentAbstract_CBF029C4AC23CADE5534CA5093D60B70" xlink:to="loc_regn_FutureExpenseOfAdvancePaymentYearOne_D507C439519DC0967538CA5093D68ADB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FutureExpenseOfAdvancePaymentYearTwo" xlink:label="loc_regn_FutureExpenseOfAdvancePaymentYearTwo_4977D21C8B4CFC4A9020CA5093D63ECC" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FutureAmortizationExpenseOfAdvancePaymentAbstract_CBF029C4AC23CADE5534CA5093D60B70" xlink:to="loc_regn_FutureExpenseOfAdvancePaymentYearTwo_4977D21C8B4CFC4A9020CA5093D63ECC" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FutureExpenseOfAdvancePaymentYearThree" xlink:label="loc_regn_FutureExpenseOfAdvancePaymentYearThree_BBDBE85B15BE19BBA05FCA5093D61C03" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FutureAmortizationExpenseOfAdvancePaymentAbstract_CBF029C4AC23CADE5534CA5093D60B70" xlink:to="loc_regn_FutureExpenseOfAdvancePaymentYearThree_BBDBE85B15BE19BBA05FCA5093D61C03" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FutureExpenseOfAdvancePaymentYearFour" xlink:label="loc_regn_FutureExpenseOfAdvancePaymentYearFour_D648ABDFB83749511CDACA5093D64F34" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FutureAmortizationExpenseOfAdvancePaymentAbstract_CBF029C4AC23CADE5534CA5093D60B70" xlink:to="loc_regn_FutureExpenseOfAdvancePaymentYearFour_D648ABDFB83749511CDACA5093D64F34" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FutureExpenseOfAdvancePaymentYearFive" xlink:label="loc_regn_FutureExpenseOfAdvancePaymentYearFive_86B6F30A34E3C9FA6F8ECA5093D668D4" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_FutureAmortizationExpenseOfAdvancePaymentAbstract_CBF029C4AC23CADE5534CA5093D60B70" xlink:to="loc_regn_FutureExpenseOfAdvancePaymentYearFive_86B6F30A34E3C9FA6F8ECA5093D668D4" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/ConsolidatedBalanceSheets" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_A2C3752D8CE94A567E34AC511979335F" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_1B79B15FEA438DCB8216AC511979375F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_A2C3752D8CE94A567E34AC511979335F" xlink:to="loc_us-gaap_StatementTable_1B79B15FEA438DCB8216AC511979375F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_5FE6E2F4968D4C594477AC511979DED2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_1B79B15FEA438DCB8216AC511979375F" xlink:to="loc_us-gaap_StatementClassOfStockAxis_5FE6E2F4968D4C594477AC511979DED2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_A7E46E3E899DE9A8A4B7AC5119799468_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_5FE6E2F4968D4C594477AC511979DED2" xlink:to="loc_us-gaap_ClassOfStockDomain_A7E46E3E899DE9A8A4B7AC5119799468_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_A7E46E3E899DE9A8A4B7AC5119799468" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_5FE6E2F4968D4C594477AC511979DED2" xlink:to="loc_us-gaap_ClassOfStockDomain_A7E46E3E899DE9A8A4B7AC5119799468" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonClassAMember" xlink:label="loc_us-gaap_CommonClassAMember_851B6658305503104400AC5119796726" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_A7E46E3E899DE9A8A4B7AC5119799468" xlink:to="loc_us-gaap_CommonClassAMember_851B6658305503104400AC5119796726" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_91774B4AB865AC6A5D7EAC511979EE23" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_A7E46E3E899DE9A8A4B7AC5119799468" xlink:to="loc_us-gaap_CommonStockMember_91774B4AB865AC6A5D7EAC511979EE23" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AssetsAbstract" xlink:label="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_A2C3752D8CE94A567E34AC511979335F" xlink:to="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:to="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_52DCBFF07960F728420CAC5119798168" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_52DCBFF07960F728420CAC5119798168" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MarketableSecuritiesCurrent" xlink:label="loc_us-gaap_MarketableSecuritiesCurrent_CC66739491F017D77AE0AC511979C77F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:to="loc_us-gaap_MarketableSecuritiesCurrent_CC66739491F017D77AE0AC511979C77F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_C3708D317DE168582120AC5119793268" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_C3708D317DE168582120AC5119793268" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DueFromRelatedPartiesCurrent" xlink:label="loc_us-gaap_DueFromRelatedPartiesCurrent_BAED0D7E1D5E3A616E4CAC511979FA88" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:to="loc_us-gaap_DueFromRelatedPartiesCurrent_BAED0D7E1D5E3A616E4CAC511979FA88" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent" xlink:label="loc_us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent_B9F9D00CA0181AD8A129AC51198339C2" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:to="loc_us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent_B9F9D00CA0181AD8A129AC51198339C2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_297AB4F5F13DE3231B22AC511983EA94" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_297AB4F5F13DE3231B22AC511983EA94" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AssetsCurrent" xlink:label="loc_us-gaap_AssetsCurrent_B66D29216CF769844532AC51198383A1" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_FD3D4A3B7FD7C01FD8EFAC511979BE1A" xlink:to="loc_us-gaap_AssetsCurrent_B66D29216CF769844532AC51198383A1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_B34721AADC7FD783E1A3AC5119839656" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_B34721AADC7FD783E1A3AC5119839656" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MarketableSecuritiesNoncurrent" xlink:label="loc_us-gaap_MarketableSecuritiesNoncurrent_9FEE3335C3F6328F90C8AC5119835B3E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:to="loc_us-gaap_MarketableSecuritiesNoncurrent_9FEE3335C3F6328F90C8AC5119835B3E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_0A9FD8A51CBC156C2233AC51198376FE" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_0A9FD8A51CBC156C2233AC51198376FE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent" xlink:label="loc_us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent_6FE432666529FBE2E3CFAC511983C619" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:to="loc_us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent_6FE432666529FBE2E3CFAC511983C619" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_5BB88B1ABD667D2AFF1AAC511983FA12" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_5BB88B1ABD667D2AFF1AAC511983FA12" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_Assets" xlink:label="loc_us-gaap_Assets_2E6890CEF351F04FD315AC511983AAFB" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_758C8702C54C12F10B51AC5119791097" xlink:to="loc_us-gaap_Assets_2E6890CEF351F04FD315AC511983AAFB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_A2C3752D8CE94A567E34AC511979335F" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_91DE8C398CBAA5A766FFAC511983E13A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_91DE8C398CBAA5A766FFAC511983E13A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" xlink:label="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent_BCCD92D4D727C38E7206AC51198370FC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_91DE8C398CBAA5A766FFAC511983E13A" xlink:to="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent_BCCD92D4D727C38E7206AC51198370FC" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueFromRelatedPartyCurrent" xlink:label="loc_regn_DeferredRevenueFromRelatedPartyCurrent_6959863A80594F7766C1AC511983FACD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_91DE8C398CBAA5A766FFAC511983E13A" xlink:to="loc_regn_DeferredRevenueFromRelatedPartyCurrent_6959863A80594F7766C1AC511983FACD" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueExcludingAmountsFromRelatedPartyCurrent" xlink:label="loc_regn_DeferredRevenueExcludingAmountsFromRelatedPartyCurrent_847BCC6C43A0AAA388ABAC5119832719" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_91DE8C398CBAA5A766FFAC511983E13A" xlink:to="loc_regn_DeferredRevenueExcludingAmountsFromRelatedPartyCurrent_847BCC6C43A0AAA388ABAC5119832719" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeaseObligationsCurrent" xlink:label="loc_regn_FacilityLeaseObligationsCurrent_D9B10C02276B4E12B23EAC511983995C" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_91DE8C398CBAA5A766FFAC511983E13A" xlink:to="loc_regn_FacilityLeaseObligationsCurrent_D9B10C02276B4E12B23EAC511983995C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:label="loc_us-gaap_LiabilitiesCurrent_88D5F3CC886E56C891BDAC511983C2EE" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_91DE8C398CBAA5A766FFAC511983E13A" xlink:to="loc_us-gaap_LiabilitiesCurrent_88D5F3CC886E56C891BDAC511983C2EE" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueFromRelatedPartyNoncurrent" xlink:label="loc_regn_DeferredRevenueFromRelatedPartyNoncurrent_EDBF879FD04BFC79B528AC5119831D3A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:to="loc_regn_DeferredRevenueFromRelatedPartyNoncurrent_EDBF879FD04BFC79B528AC5119831D3A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueExcludingAmountsFromRelatedPartyNonCurrent" xlink:label="loc_regn_DeferredRevenueExcludingAmountsFromRelatedPartyNonCurrent_12FEE00263656DE68962AC511983C64C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:to="loc_regn_DeferredRevenueExcludingAmountsFromRelatedPartyNonCurrent_12FEE00263656DE68962AC511983C64C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FacilityLeaseObligationsNoncurrent" xlink:label="loc_regn_FacilityLeaseObligationsNoncurrent_C1705B76917CA90DAED4AC511983B84D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:to="loc_regn_FacilityLeaseObligationsNoncurrent_C1705B76917CA90DAED4AC511983B84D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConvertibleLongTermNotesPayable" xlink:label="loc_us-gaap_ConvertibleLongTermNotesPayable_42F0EC1669FB6A5CAEA4AC511983D2E3" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:to="loc_us-gaap_ConvertibleLongTermNotesPayable_42F0EC1669FB6A5CAEA4AC511983D2E3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_593B38F3286F3A07BBCAAC5119834EF3" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_593B38F3286F3A07BBCAAC5119834EF3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_Liabilities" xlink:label="loc_us-gaap_Liabilities_3F86DEA30921B3438884AC5119836565" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_33FE7A6030EF4927A2FFAC5119832EC4" xlink:to="loc_us-gaap_Liabilities_3F86DEA30921B3438884AC5119836565" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="loc_us-gaap_CommitmentsAndContingencies_F92DAF218C216854D6BEAC511983EBE4" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_A2C3752D8CE94A567E34AC511979335F" xlink:to="loc_us-gaap_CommitmentsAndContingencies_F92DAF218C216854D6BEAC511983EBE4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="loc_us-gaap_StockholdersEquityAbstract_95BBD145B78659C33054AC5119839670" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_A2C3752D8CE94A567E34AC511979335F" xlink:to="loc_us-gaap_StockholdersEquityAbstract_95BBD145B78659C33054AC5119839670" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PreferredStockValue" xlink:label="loc_us-gaap_PreferredStockValue_E3CD62A96CAC6DDCF9ABAC5119833C2C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_95BBD145B78659C33054AC5119839670" xlink:to="loc_us-gaap_PreferredStockValue_E3CD62A96CAC6DDCF9ABAC5119833C2C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockValue" xlink:label="loc_us-gaap_CommonStockValue_85E462CFC792E837EEE4AC511983E323" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_95BBD145B78659C33054AC5119839670" xlink:to="loc_us-gaap_CommonStockValue_85E462CFC792E837EEE4AC511983E323" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdditionalPaidInCapital" xlink:label="loc_us-gaap_AdditionalPaidInCapital_3AD1D7986C87E4CE126BAC511983CDD4" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_95BBD145B78659C33054AC5119839670" xlink:to="loc_us-gaap_AdditionalPaidInCapital_3AD1D7986C87E4CE126BAC511983CDD4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_FDDFB03B9017FCCC6D6DAC51198326B3" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_95BBD145B78659C33054AC5119839670" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_FDDFB03B9017FCCC6D6DAC51198326B3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_7A1E6A077EE18286160CAC51198346A1" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_95BBD145B78659C33054AC5119839670" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_7A1E6A077EE18286160CAC51198346A1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_B662597AC8FA07894738AC5119835729" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_95BBD145B78659C33054AC5119839670" xlink:to="loc_us-gaap_StockholdersEquity_B662597AC8FA07894738AC5119835729" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_94E5052FA3478BD5BE14AC511983F90A" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_A2C3752D8CE94A567E34AC511979335F" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_94E5052FA3478BD5BE14AC511983F90A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_9E2E6AC0777A489A3902898D57FB4844" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_C4211B465248B4602D08898D57FAC1FE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_9E2E6AC0777A489A3902898D57FB4844" xlink:to="loc_us-gaap_StatementTable_C4211B465248B4602D08898D57FAC1FE" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_9761A6D522D622A7F907898D57FA790B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_C4211B465248B4602D08898D57FAC1FE" xlink:to="loc_us-gaap_StatementClassOfStockAxis_9761A6D522D622A7F907898D57FA790B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_4AB313C130238940FBDA898D57FB1FF8_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_9761A6D522D622A7F907898D57FA790B" xlink:to="loc_us-gaap_ClassOfStockDomain_4AB313C130238940FBDA898D57FB1FF8_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_4AB313C130238940FBDA898D57FB1FF8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_9761A6D522D622A7F907898D57FA790B" xlink:to="loc_us-gaap_ClassOfStockDomain_4AB313C130238940FBDA898D57FB1FF8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonClassAMember" xlink:label="loc_us-gaap_CommonClassAMember_2575B829C074D183D4FC898D57FB9D82" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_4AB313C130238940FBDA898D57FB1FF8" xlink:to="loc_us-gaap_CommonClassAMember_2575B829C074D183D4FC898D57FB9D82" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_CB981B9F282649F9F067898D57FB7B4C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_4AB313C130238940FBDA898D57FB1FF8" xlink:to="loc_us-gaap_CommonStockMember_CB981B9F282649F9F067898D57FB7B4C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_806A2260C45A19E1E40B898D57FB673B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_9E2E6AC0777A489A3902898D57FB4844" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_806A2260C45A19E1E40B898D57FB673B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_806A2260C45A19E1E40B898D57FB673B" xlink:to="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_75776298E3C9781A64D7898D57FB0C83" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_75776298E3C9781A64D7898D57FB0C83" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_99650E6AD02CD4158285898D57FB76BA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_99650E6AD02CD4158285898D57FB76BA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="loc_us-gaap_PreferredStockSharesIssued_B3D70DD88ED2A6CE0143898D57FC7E66" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:to="loc_us-gaap_PreferredStockSharesIssued_B3D70DD88ED2A6CE0143898D57FC7E66" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_BAC206FA7ABE5AAEF727898D57FC0120" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_BAC206FA7ABE5AAEF727898D57FC0120" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_3C6A0C56BBA980DBDE78898D57FCDAF1" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_3C6A0C56BBA980DBDE78898D57FCDAF1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_57FF65BF427E737745D4898D57FCE46B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_57FF65BF427E737745D4898D57FCE46B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:label="loc_us-gaap_CommonStockSharesIssued_81E63408C7A52EBF5806898D57FCCC93" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:to="loc_us-gaap_CommonStockSharesIssued_81E63408C7A52EBF5806898D57FCCC93" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_F0A5CF81C2EE58BCB986898D57FCF5DA" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_9D5C590CFA161B3E62F0898D57FBF4F7" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_F0A5CF81C2EE58BCB986898D57FCF5DA" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/ConsolidatedStatementsOfCashFlows" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_918E0ABD155B06C6BB0DB0809834B2FF" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_6A5311EEBB29A450EE87B08098345475" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_918E0ABD155B06C6BB0DB0809834B2FF" xlink:to="loc_us-gaap_StatementTable_6A5311EEBB29A450EE87B08098345475" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_3AD7549136ECFE33AFBBB0809834D90C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_6A5311EEBB29A450EE87B08098345475" xlink:to="loc_us-gaap_StatementScenarioAxis_3AD7549136ECFE33AFBBB0809834D90C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_6F1405ACA95715B18B0EB0809834081C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_3AD7549136ECFE33AFBBB0809834D90C" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_6F1405ACA95715B18B0EB0809834081C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_6F1405ACA95715B18B0EB0809834081C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_3AD7549136ECFE33AFBBB0809834D90C" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_6F1405ACA95715B18B0EB0809834081C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1016E7F12B50EDEA5017B0809834D33A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_918E0ABD155B06C6BB0DB0809834B2FF" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1016E7F12B50EDEA5017B0809834D33A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_1DE4358EA6BC2590676FB0809834E4DC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1016E7F12B50EDEA5017B0809834D33A" xlink:to="loc_us-gaap_NetIncomeLoss_1DE4358EA6BC2590676FB0809834E4DC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1016E7F12B50EDEA5017B0809834D33A" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_5B93634C77A90A05DE69B0809834320B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_5B93634C77A90A05DE69B0809834320B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensation" xlink:label="loc_us-gaap_ShareBasedCompensation_6162E2B4450533AB168BB080983E016A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:to="loc_us-gaap_ShareBasedCompensation_6162E2B4450533AB168BB080983E016A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NonCashInterestExpense" xlink:label="loc_regn_NonCashInterestExpense_55AD64ECB7B986307F48B080983E6107" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:to="loc_regn_NonCashInterestExpense_55AD64ECB7B986307F48B080983E6107" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OtherNoncashIncomeExpense" xlink:label="loc_us-gaap_OtherNoncashIncomeExpense_6C1825987347B8745412B080983E1CE3" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:to="loc_us-gaap_OtherNoncashIncomeExpense_6C1825987347B8745412B080983E1CE3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_0B7FEF42613C14B2C709B080983EADD1" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_0B7FEF42613C14B2C709B080983EADD1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_045AD90563AB5CE25261B080983ECBAE" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_045AD90563AB5CE25261B080983ECBAE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_18C4936E128384252541B080983EF1D0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_045AD90563AB5CE25261B080983ECBAE" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_18C4936E128384252541B080983EF1D0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_D7E725CD028FF3B7F1C2B080983E07EC" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_045AD90563AB5CE25261B080983ECBAE" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_D7E725CD028FF3B7F1C2B080983E07EC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncreaseDecreaseInDeferredRevenue" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredRevenue_E3FD7363ADD845BF09A2B080983ED2A1" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_045AD90563AB5CE25261B080983ECBAE" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredRevenue_E3FD7363ADD845BF09A2B080983ED2A1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_CBF23BDF010A4EB2D5F4B080983E2541" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_045AD90563AB5CE25261B080983ECBAE" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_CBF23BDF010A4EB2D5F4B080983E2541" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivities" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivities_0506E1AFE94D598751DDB080983ECE92" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_443F6493C95A1B30AC1BB0809834590F" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivities_0506E1AFE94D598751DDB080983ECE92" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_A8C741956ADC73D5957DB080983EFC24" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1016E7F12B50EDEA5017B0809834D33A" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_A8C741956ADC73D5957DB080983EFC24" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_F67C3CEB1C9A3F907AEBB080983EB0CD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_918E0ABD155B06C6BB0DB0809834B2FF" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_F67C3CEB1C9A3F907AEBB080983EB0CD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PaymentsToAcquireMarketableSecurities" xlink:label="loc_us-gaap_PaymentsToAcquireMarketableSecurities_980D86E67B307EA4BF28B080983E7D4D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_F67C3CEB1C9A3F907AEBB080983EB0CD" xlink:to="loc_us-gaap_PaymentsToAcquireMarketableSecurities_980D86E67B307EA4BF28B080983E7D4D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities" xlink:label="loc_us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities_5AD4C296455048325B37B080983EEFF3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_F67C3CEB1C9A3F907AEBB080983EB0CD" xlink:to="loc_us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities_5AD4C296455048325B37B080983EEFF3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncreaseDecreaseInRestrictedCashAndInvestments" xlink:label="loc_us-gaap_IncreaseDecreaseInRestrictedCashAndInvestments_AD5FDF245C5D8E6A7BEAB080983E7E32" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_F67C3CEB1C9A3F907AEBB080983EB0CD" xlink:to="loc_us-gaap_IncreaseDecreaseInRestrictedCashAndInvestments_AD5FDF245C5D8E6A7BEAB080983E7E32" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PaymentsToAcquireProductiveAssets" xlink:label="loc_us-gaap_PaymentsToAcquireProductiveAssets_880B4B1A2AAF328A6C83B080983EB0FB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_F67C3CEB1C9A3F907AEBB080983EB0CD" xlink:to="loc_us-gaap_PaymentsToAcquireProductiveAssets_880B4B1A2AAF328A6C83B080983EB0FB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_343C481AA3EFD062C574B080983E1F4B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_F67C3CEB1C9A3F907AEBB080983EB0CD" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_343C481AA3EFD062C574B080983E1F4B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_918E0ABD155B06C6BB0DB0809834B2FF" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromLongTermCapitalLeaseObligations" xlink:label="loc_us-gaap_ProceedsFromLongTermCapitalLeaseObligations_F300724C9C0EA9D74879B080983EE50E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_ProceedsFromLongTermCapitalLeaseObligations_F300724C9C0EA9D74879B080983EE50E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RepaymentsOfLongTermCapitalLeaseObligations" xlink:label="loc_us-gaap_RepaymentsOfLongTermCapitalLeaseObligations_4563C8CD8F2C53071540B080983EA277" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_RepaymentsOfLongTermCapitalLeaseObligations_4563C8CD8F2C53071540B080983EA277" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromConvertibleDebt" xlink:label="loc_us-gaap_ProceedsFromConvertibleDebt_AC6110E5A42D32B37FF1B080983E0307" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_ProceedsFromConvertibleDebt_AC6110E5A42D32B37FF1B080983E0307" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromIssuanceOfWarrants" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfWarrants_5CA525CE7EA30678FD0CB080983EA497" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfWarrants_5CA525CE7EA30678FD0CB080983EA497" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PaymentsForHedgeFinancingActivities" xlink:label="loc_us-gaap_PaymentsForHedgeFinancingActivities_7598AB3460F7FB6FC647B080983E247C" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_PaymentsForHedgeFinancingActivities_7598AB3460F7FB6FC647B080983E247C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_B492D1E902F97775DBFEB080983E44E7" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_B492D1E902F97775DBFEB080983E44E7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_30108C7B38191422591BB080983EAD0A" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_30108C7B38191422591BB080983EAD0A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities" xlink:label="loc_us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities_64BEC95ADF41A6F0DDB6B080983EE557" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities_64BEC95ADF41A6F0DDB6B080983EE557" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_65ECF90CB9C34EAEF96FB080983E7304" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_C4A050A05050DE48676BB080983E620D" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_65ECF90CB9C34EAEF96FB080983E7304" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease" xlink:label="loc_us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease_B9014E30EB06C532DA52B080983EDEEB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_918E0ABD155B06C6BB0DB0809834B2FF" xlink:to="loc_us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease_B9014E30EB06C532DA52B080983EDEEB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_2C68A88A5D9EFCD0DD9EB080983EA7D9" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_918E0ABD155B06C6BB0DB0809834B2FF" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_2C68A88A5D9EFCD0DD9EB080983EA7D9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SupplementalCashFlowInformationAbstract" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_2FC45624B282C193366EB080983EA9FF" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_918E0ABD155B06C6BB0DB0809834B2FF" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_2FC45624B282C193366EB080983EA9FF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_InterestPaidNet" xlink:label="loc_us-gaap_InterestPaidNet_86094E28BCE99AF3A14BB080983E6ED6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_2FC45624B282C193366EB080983EA9FF" xlink:to="loc_us-gaap_InterestPaidNet_86094E28BCE99AF3A14BB080983E6ED6" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveIncomeLoss" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_D6AF4E0E3D177B34D3EDCC080A4B7F99" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_2BCF4EA57CA73CF831E2CC080A4B581E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_D6AF4E0E3D177B34D3EDCC080A4B7F99" xlink:to="loc_us-gaap_StatementTable_2BCF4EA57CA73CF831E2CC080A4B581E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_7AED4ECE3C2A819A2C5CCC080A4B96FE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_2BCF4EA57CA73CF831E2CC080A4B581E" xlink:to="loc_us-gaap_StatementScenarioAxis_7AED4ECE3C2A819A2C5CCC080A4B96FE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_F16F7FDB67735139DC2FCC080A4B44B5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_7AED4ECE3C2A819A2C5CCC080A4B96FE" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_F16F7FDB67735139DC2FCC080A4B44B5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_F16F7FDB67735139DC2FCC080A4B44B5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_7AED4ECE3C2A819A2C5CCC080A4B96FE" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_F16F7FDB67735139DC2FCC080A4B44B5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncomeStatementAbstract" xlink:label="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_D6AF4E0E3D177B34D3EDCC080A4B7F99" xlink:to="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RevenuesAbstract" xlink:label="loc_us-gaap_RevenuesAbstract_22166F4769807511F814CC080A4BB218" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_RevenuesAbstract_22166F4769807511F814CC080A4BB218" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_B7F769F8CA7EBA74D572CC080A4B1696" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_22166F4769807511F814CC080A4BB218" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_B7F769F8CA7EBA74D572CC080A4B1696" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RevenueFromRelatedParties" xlink:label="loc_us-gaap_RevenueFromRelatedParties_A7E34954D887ECEF745CCC080A4CE672" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_22166F4769807511F814CC080A4BB218" xlink:to="loc_us-gaap_RevenueFromRelatedParties_A7E34954D887ECEF745CCC080A4CE672" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_OtherCollaborationRevenue" xlink:label="loc_regn_OtherCollaborationRevenue_A0D16216F9D2791CFD61CC080A4C966D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_22166F4769807511F814CC080A4BB218" xlink:to="loc_regn_OtherCollaborationRevenue_A0D16216F9D2791CFD61CC080A4C966D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LicensesRevenue" xlink:label="loc_us-gaap_LicensesRevenue_0E2160452BA9FF5397F0CC080A4CBA0B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_22166F4769807511F814CC080A4BB218" xlink:to="loc_us-gaap_LicensesRevenue_0E2160452BA9FF5397F0CC080A4CBA0B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ContractsRevenue" xlink:label="loc_us-gaap_ContractsRevenue_F009C984EDF606736994CC080A4CB87B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_22166F4769807511F814CC080A4BB218" xlink:to="loc_us-gaap_ContractsRevenue_F009C984EDF606736994CC080A4CB87B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_320B22917073D1F9554DCC080A4C4E4A" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_22166F4769807511F814CC080A4BB218" xlink:to="loc_us-gaap_Revenues_320B22917073D1F9554DCC080A4C4E4A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CostsAndExpensesAbstract" xlink:label="loc_us-gaap_CostsAndExpensesAbstract_F03EAE9AD4FAFDE6CCF8CC080A4C4FF8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_CostsAndExpensesAbstract_F03EAE9AD4FAFDE6CCF8CC080A4C4FF8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_38FE78A1A23086B36423CC080A4CCF41" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_F03EAE9AD4FAFDE6CCF8CC080A4C4FF8" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_38FE78A1A23086B36423CC080A4CCF41" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpense" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_9D883D89CEB4492E341DCC080A4CE457" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_F03EAE9AD4FAFDE6CCF8CC080A4C4FF8" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_9D883D89CEB4492E341DCC080A4CE457" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CostOfGoodsSold" xlink:label="loc_us-gaap_CostOfGoodsSold_7507A968A3430CE29BC2CC080A4C2D1D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_F03EAE9AD4FAFDE6CCF8CC080A4C4FF8" xlink:to="loc_us-gaap_CostOfGoodsSold_7507A968A3430CE29BC2CC080A4C2D1D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CostsAndExpenses" xlink:label="loc_us-gaap_CostsAndExpenses_8FECE1F46EC119E1BFDDCC080A4CDA04" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_F03EAE9AD4FAFDE6CCF8CC080A4C4FF8" xlink:to="loc_us-gaap_CostsAndExpenses_8FECE1F46EC119E1BFDDCC080A4CDA04" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="loc_us-gaap_OperatingIncomeLoss_54A6E0CDE7B1967BE137CC080A4C9037" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_OperatingIncomeLoss_54A6E0CDE7B1967BE137CC080A4C9037" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NonoperatingIncomeExpenseAbstract" xlink:label="loc_us-gaap_NonoperatingIncomeExpenseAbstract_56E836C501F1B8F06E45CC080A4C64BA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_NonoperatingIncomeExpenseAbstract_56E836C501F1B8F06E45CC080A4C64BA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_InvestmentIncomeNonoperating" xlink:label="loc_us-gaap_InvestmentIncomeNonoperating_3D86D6C0D579717B852ACC080A4CEDA1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_56E836C501F1B8F06E45CC080A4C64BA" xlink:to="loc_us-gaap_InvestmentIncomeNonoperating_3D86D6C0D579717B852ACC080A4CEDA1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_InterestExpense" xlink:label="loc_us-gaap_InterestExpense_4AC1F130CAF7C42D826FCC080A4D0EFA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_56E836C501F1B8F06E45CC080A4C64BA" xlink:to="loc_us-gaap_InterestExpense_4AC1F130CAF7C42D826FCC080A4D0EFA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_B7E0776282E0822850E3CC080A4D1D00" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_56E836C501F1B8F06E45CC080A4C64BA" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_B7E0776282E0822850E3CC080A4D1D00" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_DE986CD2E6366175392CCC080A4D5B69" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_DE986CD2E6366175392CCC080A4D5B69" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_ADB7B75B82FB7C77A648CC080A4D0900" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_ADB7B75B82FB7C77A648CC080A4D0900" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_A3AF10CABE4A6B37F889CC080A4D3E4E" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_NetIncomeLoss_A3AF10CABE4A6B37F889CC080A4D3E4E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EarningsPerShareBasic" xlink:label="loc_us-gaap_EarningsPerShareBasic_5CBFEC841D07DF5209D7CC080A4D0341" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_EarningsPerShareBasic_5CBFEC841D07DF5209D7CC080A4D0341" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="loc_us-gaap_EarningsPerShareDiluted_B20CD9D587A3AA83D865CC080A4D106E" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_EarningsPerShareDiluted_B20CD9D587A3AA83D865CC080A4D106E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_9A3A507029244ABBD528CC080A4D35B3" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_9A3A507029244ABBD528CC080A4D35B3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_8F7B9D517368127ABC00CC080A4D5E70" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract_28E151999C659639CE82CC080A4BB6C8" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_8F7B9D517368127ABC00CC080A4D5E70" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTaxAbstract" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_D57D349F3B6520CF7BCDCC080A4D9AA6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_D6AF4E0E3D177B34D3EDCC080A4B7F99" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_D57D349F3B6520CF7BCDCC080A4D9AA6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_8F6E4A366AD8ABC23BC9CC080A4DD087" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_D57D349F3B6520CF7BCDCC080A4D9AA6" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_8F6E4A366AD8ABC23BC9CC080A4DD087" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_560141A3D3DE53755F47CC080A4D3E4D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_8F6E4A366AD8ABC23BC9CC080A4DD087" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_560141A3D3DE53755F47CC080A4D3E4D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTax" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_842876A59F25DE2E1C60CC080A4D84C8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_D57D349F3B6520CF7BCDCC080A4D9AA6" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_842876A59F25DE2E1C60CC080A4D84C8" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/ConsolidatedStatementsOfStockholdersEquity" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_F0DDEE637599DA816B74D57BFE45B308" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_FDB54330A3879EAFCDE0D57BFE452D56" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_F0DDEE637599DA816B74D57BFE45B308" xlink:to="loc_us-gaap_StatementTable_FDB54330A3879EAFCDE0D57BFE452D56" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_E61997EFEC623D4E2EEBD57BFE451F2B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_FDB54330A3879EAFCDE0D57BFE452D56" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_E61997EFEC623D4E2EEBD57BFE451F2B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_E61997EFEC623D4E2EEBD57BFE451F2B" xlink:to="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_E61997EFEC623D4E2EEBD57BFE451F2B" xlink:to="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonClassAMember" xlink:label="loc_us-gaap_CommonClassAMember_AEF5E5119F7567B5CDD9D57BFE45F341" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65" xlink:to="loc_us-gaap_CommonClassAMember_AEF5E5119F7567B5CDD9D57BFE45F341" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_0F216CB8AEC974CF865BD57BFE4584C9" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65" xlink:to="loc_us-gaap_CommonStockMember_0F216CB8AEC974CF865BD57BFE4584C9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_FD04605D519D6C3539D4D57BFE457AEC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_FD04605D519D6C3539D4D57BFE457AEC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_0A5D70CFABFD2C807F9AD57BFE45A59C" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65" xlink:to="loc_us-gaap_RetainedEarningsMember_0A5D70CFABFD2C807F9AD57BFE45A59C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_768D33CD66FC482869C7D57BFE4560ED" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D16563485ECCF470200BD57BFE45AD65" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_768D33CD66FC482869C7D57BFE4560ED" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_F0DDEE637599DA816B74D57BFE45B308" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_FDE348716E65F8AB2883D57BFE45D50B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockholdersEquity_FDE348716E65F8AB2883D57BFE45D50B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SharesOutstanding" xlink:label="loc_us-gaap_SharesOutstanding_282E942CE053DFC5B8DFD57BFE45763B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_SharesOutstanding_282E942CE053DFC5B8DFD57BFE45763B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_A6D894E63EBF9123A75AD57BFE456360" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_A6D894E63EBF9123A75AD57BFE456360" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_4BADC16ECB83AE9CAB15D57BFE45F804" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_4BADC16ECB83AE9CAB15D57BFE45F804" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_8A5980F7A2CCDAD37546D57BFE45F546" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_8A5980F7A2CCDAD37546D57BFE45F546" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_25410587009B3FCAFB8AD57BFE468089" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_25410587009B3FCAFB8AD57BFE468089" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CommonStockSharesPaidForTaxWithholdingForShareBasedCompensation" xlink:label="loc_regn_CommonStockSharesPaidForTaxWithholdingForShareBasedCompensation_E7647D220771F8479B0AD57BFE462D47" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_regn_CommonStockSharesPaidForTaxWithholdingForShareBasedCompensation_E7647D220771F8479B0AD57BFE462D47" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensationStockOptions" xlink:label="loc_regn_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensationStockOptions_55BB4DCD301A49E2FF02D57BFE46B419" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_regn_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensationStockOptions_55BB4DCD301A49E2FF02D57BFE46B419" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan_0F050F59EF136774D39FD57BFE466CCE" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan_0F050F59EF136774D39FD57BFE466CCE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeBenefitPlan" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeBenefitPlan_11BBB6C2DF5A653EE729D57BFE46DACC" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeBenefitPlan_11BBB6C2DF5A653EE729D57BFE46DACC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_F67947FD01DAA3DF1CEED57BFE46159C" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_F67947FD01DAA3DF1CEED57BFE46159C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensationRestrictedStock" xlink:label="loc_regn_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensationRestrictedStock_C2DAF9B02BFA0F0F4810D57BFE465469" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_regn_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensationRestrictedStock_C2DAF9B02BFA0F0F4810D57BFE465469" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RestrictedCommonStockSharesPaidForTaxWithholdingForShareBasedCompensation" xlink:label="loc_regn_RestrictedCommonStockSharesPaidForTaxWithholdingForShareBasedCompensation_8E720BC43306F9F14D7CD57BFE46AFD5" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_regn_RestrictedCommonStockSharesPaidForTaxWithholdingForShareBasedCompensation_8E720BC43306F9F14D7CD57BFE46AFD5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_0E3CD81474CBBE923F38D57BFE465678" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_0E3CD81474CBBE923F38D57BFE465678" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_9428316C9DCCA969AA2AD57BFE4649AE" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_9428316C9DCCA969AA2AD57BFE4649AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet" xlink:label="loc_us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet_1DC523344BFF6FDA0DAED57BFE46F313" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet_1DC523344BFF6FDA0DAED57BFE46F313" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebt" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebt_52F6399F2D062CAE9E39D57BFE46E9F1" xlink:type="locator" />
    <link:definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebt_52F6399F2D062CAE9E39D57BFE46E9F1" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AdjustmentToAdditionalPaidInCapitalCostOfConvertibleNoteHedge" xlink:label="loc_regn_AdjustmentToAdditionalPaidInCapitalCostOfConvertibleNoteHedge_57D92FD98E6A262FD67FD57BFE46CA71" xlink:type="locator" />
    <link:definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_regn_AdjustmentToAdditionalPaidInCapitalCostOfConvertibleNoteHedge_57D92FD98E6A262FD67FD57BFE46CA71" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_78945DD433B2AA6BCF82D57BFE4675BC" xlink:type="locator" />
    <link:definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_78945DD433B2AA6BCF82D57BFE4675BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_B13CDDC88416128E872CD57BFE46073B" xlink:type="locator" />
    <link:definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_NetIncomeLoss_B13CDDC88416128E872CD57BFE46073B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_B3D523A046F418E42986D57BFE46823D" xlink:type="locator" />
    <link:definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_10CB7C2D76FDE5177235D57BFE453447" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_B3D523A046F418E42986D57BFE46823D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/ConvertibleDebtDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_2B2A5E093332E2C7D635CEC3E194B2F5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_DebtInstrumentTable_2B2A5E093332E2C7D635CEC3E194B2F5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_8BBFCDBDB06156694696CEC3E1948857" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_2B2A5E093332E2C7D635CEC3E194B2F5" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_8BBFCDBDB06156694696CEC3E1948857" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9535A143EA430703AE63CEC3E194C6A4_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_8BBFCDBDB06156694696CEC3E1948857" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9535A143EA430703AE63CEC3E194C6A4_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9535A143EA430703AE63CEC3E194C6A4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_8BBFCDBDB06156694696CEC3E1948857" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9535A143EA430703AE63CEC3E194C6A4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConvertibleDebtMember" xlink:label="loc_us-gaap_ConvertibleDebtMember_E8F95F1A925E5670E0EECEC3E194C0E6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9535A143EA430703AE63CEC3E194C6A4" xlink:to="loc_us-gaap_ConvertibleDebtMember_E8F95F1A925E5670E0EECEC3E194C0E6" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentRedemptionPeriodAxis" xlink:label="loc_regn_DebtInstrumentRedemptionPeriodAxis_663CA72BB77F4393A5D4CEC3E1943FDE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_2B2A5E093332E2C7D635CEC3E194B2F5" xlink:to="loc_regn_DebtInstrumentRedemptionPeriodAxis_663CA72BB77F4393A5D4CEC3E1943FDE" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentRedemptionPeriodDomain" xlink:label="loc_regn_DebtInstrumentRedemptionPeriodDomain_BA6629ED55AE7C92C0BECEC3E1940981_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_DebtInstrumentRedemptionPeriodAxis_663CA72BB77F4393A5D4CEC3E1943FDE" xlink:to="loc_regn_DebtInstrumentRedemptionPeriodDomain_BA6629ED55AE7C92C0BECEC3E1940981_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentRedemptionPeriodDomain" xlink:label="loc_regn_DebtInstrumentRedemptionPeriodDomain_BA6629ED55AE7C92C0BECEC3E1940981" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_DebtInstrumentRedemptionPeriodAxis_663CA72BB77F4393A5D4CEC3E1943FDE" xlink:to="loc_regn_DebtInstrumentRedemptionPeriodDomain_BA6629ED55AE7C92C0BECEC3E1940981" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentRedemptionPeriodOneMember" xlink:label="loc_regn_DebtInstrumentRedemptionPeriodOneMember_45448C8E8954F9E90B92CEC3E194B1F4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_DebtInstrumentRedemptionPeriodDomain_BA6629ED55AE7C92C0BECEC3E1940981" xlink:to="loc_regn_DebtInstrumentRedemptionPeriodOneMember_45448C8E8954F9E90B92CEC3E194B1F4" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentRedemptionPeriodTwoMember" xlink:label="loc_regn_DebtInstrumentRedemptionPeriodTwoMember_C10B35C1337E63DDB4F7CEC3E1941914" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_DebtInstrumentRedemptionPeriodDomain_BA6629ED55AE7C92C0BECEC3E1940981" xlink:to="loc_regn_DebtInstrumentRedemptionPeriodTwoMember_C10B35C1337E63DDB4F7CEC3E1941914" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentRedemptionPeriodThreeMember" xlink:label="loc_regn_DebtInstrumentRedemptionPeriodThreeMember_8DDECF0CF79A0E9ACB42CEC3E194D06E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_DebtInstrumentRedemptionPeriodDomain_BA6629ED55AE7C92C0BECEC3E1940981" xlink:to="loc_regn_DebtInstrumentRedemptionPeriodThreeMember_8DDECF0CF79A0E9ACB42CEC3E194D06E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeAxis" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_24B580728C42255F83D5CEC3E194E862" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_2B2A5E093332E2C7D635CEC3E194B2F5" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_24B580728C42255F83D5CEC3E194E862" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeDomain" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_A3304CAC19F25B007E1CCEC3E1943730_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_24B580728C42255F83D5CEC3E194E862" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_A3304CAC19F25B007E1CCEC3E1943730_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeDomain" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_A3304CAC19F25B007E1CCEC3E1943730" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_24B580728C42255F83D5CEC3E194E862" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_A3304CAC19F25B007E1CCEC3E1943730" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CallOptionMember" xlink:label="loc_us-gaap_CallOptionMember_4CF17352FED4D6171655CEC3E1946EEB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_A3304CAC19F25B007E1CCEC3E1943730" xlink:to="loc_us-gaap_CallOptionMember_4CF17352FED4D6171655CEC3E1946EEB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_B21F3218A3D381B192A3CEC3E19EBDCB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_2B2A5E093332E2C7D635CEC3E194B2F5" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_B21F3218A3D381B192A3CEC3E19EBDCB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_8C53E47F5E95BA6EE179CEC3E19E6638_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_B21F3218A3D381B192A3CEC3E19EBDCB" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_8C53E47F5E95BA6EE179CEC3E19E6638_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_8C53E47F5E95BA6EE179CEC3E19E6638" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_B21F3218A3D381B192A3CEC3E19EBDCB" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_8C53E47F5E95BA6EE179CEC3E19E6638" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_WarrantTransactionsMember" xlink:label="loc_regn_WarrantTransactionsMember_C562021216A629387F3FCEC3E19E2797" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_8C53E47F5E95BA6EE179CEC3E19E6638" xlink:to="loc_regn_WarrantTransactionsMember_C562021216A629387F3FCEC3E19E2797" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_9B68EC730F9FFCC0928FCEC3E19E5DCA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_9B68EC730F9FFCC0928FCEC3E19E5DCA" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ProceedsFromConvertibleDebtNetOfIssuanceCosts" xlink:label="loc_regn_ProceedsFromConvertibleDebtNetOfIssuanceCosts_A881E467FFEE5596512ACEC3E19E9152" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_ProceedsFromConvertibleDebtNetOfIssuanceCosts_A881E467FFEE5596512ACEC3E19E9152" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentConvertibleConversionRatioSharesPer1000OfPrincipal" xlink:label="loc_regn_DebtInstrumentConvertibleConversionRatioSharesPer1000OfPrincipal_F0FB80ABBCFD74BE21A7CEC3E19E4948" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentConvertibleConversionRatioSharesPer1000OfPrincipal_F0FB80ABBCFD74BE21A7CEC3E19E4948" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentConvertibleIncrementOfPrincipalAmountOfConversion" xlink:label="loc_regn_DebtInstrumentConvertibleIncrementOfPrincipalAmountOfConversion_D366BC34E023686B9C9ECEC3E19E77C4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentConvertibleIncrementOfPrincipalAmountOfConversion_D366BC34E023686B9C9ECEC3E19E77C4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentConvertibleNumberOfEquityInstruments" xlink:label="loc_us-gaap_DebtInstrumentConvertibleNumberOfEquityInstruments_56363B941797386B6448CEC3E19E83B6" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_DebtInstrumentConvertibleNumberOfEquityInstruments_56363B941797386B6448CEC3E19E83B6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_6E4B65F5DA714D1F5348CEC3E19EBFC9" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_6E4B65F5DA714D1F5348CEC3E19EBFC9" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentConvertibleThresholdTradingDays" xlink:label="loc_regn_DebtInstrumentConvertibleThresholdTradingDays_332F4D6EF85245DBEA4BCEC3E19E523E" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentConvertibleThresholdTradingDays_332F4D6EF85245DBEA4BCEC3E19E523E" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentConvertibleThresholdConsecutiveTradingDays" xlink:label="loc_regn_DebtInstrumentConvertibleThresholdConsecutiveTradingDays_00F3A8BCF89E5AF14BB0CEC3E19EAE29" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentConvertibleThresholdConsecutiveTradingDays_00F3A8BCF89E5AF14BB0CEC3E19EAE29" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger" xlink:label="loc_regn_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_5EEC75D89B219CE33C93CEC3E19EFEAE" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_5EEC75D89B219CE33C93CEC3E19EFEAE" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentConvertibleCalendarDaysAfterRecordDate" xlink:label="loc_regn_DebtInstrumentConvertibleCalendarDaysAfterRecordDate_6E5888FD3F508BA9DC5ECEC3E19E3A00" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentConvertibleCalendarDaysAfterRecordDate_6E5888FD3F508BA9DC5ECEC3E19E3A00" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentConvertibleThresholdConsecutiveDays" xlink:label="loc_regn_DebtInstrumentConvertibleThresholdConsecutiveDays_016BC9B3F0A31E34DA2ACEC3E19E18A7" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentConvertibleThresholdConsecutiveDays_016BC9B3F0A31E34DA2ACEC3E19E18A7" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentPercentageOfPrincipalValueThatHoldersCanRequireCompanyToRepurchase" xlink:label="loc_regn_DebtInstrumentPercentageOfPrincipalValueThatHoldersCanRequireCompanyToRepurchase_30A69DBFCCBB251DE125CEC3E19E62C5" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentPercentageOfPrincipalValueThatHoldersCanRequireCompanyToRepurchase_30A69DBFCCBB251DE125CEC3E19E62C5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentConvertibleIfConvertedValueInExcessOfPrincipal" xlink:label="loc_us-gaap_DebtInstrumentConvertibleIfConvertedValueInExcessOfPrincipal_B93DEBDF5533D655F871CEC3E1A8081F" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_DebtInstrumentConvertibleIfConvertedValueInExcessOfPrincipal_B93DEBDF5533D655F871CEC3E1A8081F" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DebtInstrumentConvertibleFairValueOfLiabilityComponent" xlink:label="loc_regn_DebtInstrumentConvertibleFairValueOfLiabilityComponent_520E79A7D1C2B9EB1664CEC3E1A8526E" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_DebtInstrumentConvertibleFairValueOfLiabilityComponent_520E79A7D1C2B9EB1664CEC3E1A8526E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentConvertibleCarryingAmountOfTheEquityComponent" xlink:label="loc_us-gaap_DebtInstrumentConvertibleCarryingAmountOfTheEquityComponent_9EFFC5E5E316B78E7169CEC3E1A8B557" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_DebtInstrumentConvertibleCarryingAmountOfTheEquityComponent_9EFFC5E5E316B78E7169CEC3E1A8B557" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_73979CE17012B800E6C5CEC3E6132FBE" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_73979CE17012B800E6C5CEC3E6132FBE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentConvertibleEffectiveInterestRate" xlink:label="loc_us-gaap_DebtInstrumentConvertibleEffectiveInterestRate_560DFD38C15CE5D303E2CEC3E1A8CE18" xlink:type="locator" />
    <link:definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_DebtInstrumentConvertibleEffectiveInterestRate_560DFD38C15CE5D303E2CEC3E1A8CE18" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_IssuanceCosts" xlink:label="loc_regn_IssuanceCosts_B9D762AA17CD9ADE0004CEC3E1A8B1BD" xlink:type="locator" />
    <link:definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_IssuanceCosts_B9D762AA17CD9ADE0004CEC3E1A8B1BD" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NetCarryingAmountOfLiabilityComponentAbstract" xlink:label="loc_regn_NetCarryingAmountOfLiabilityComponentAbstract_6FABBADE57FD382B91C2CEC3E1A894D7" xlink:type="locator" />
    <link:definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_regn_NetCarryingAmountOfLiabilityComponentAbstract_6FABBADE57FD382B91C2CEC3E1A894D7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_ADBF430563DA7EBA9391CEC3E1A83051" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_NetCarryingAmountOfLiabilityComponentAbstract_6FABBADE57FD382B91C2CEC3E1A894D7" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_ADBF430563DA7EBA9391CEC3E1A83051" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentUnamortizedDiscount" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_60CB6EE3274ECBAB9917CEC3E1A84E35" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_NetCarryingAmountOfLiabilityComponentAbstract_6FABBADE57FD382B91C2CEC3E1A894D7" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_60CB6EE3274ECBAB9917CEC3E1A84E35" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongTermDebt" xlink:label="loc_us-gaap_LongTermDebt_AC7F7F0E9CEECE01A339CEC3E1A8FA30" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_NetCarryingAmountOfLiabilityComponentAbstract_6FABBADE57FD382B91C2CEC3E1A894D7" xlink:to="loc_us-gaap_LongTermDebt_AC7F7F0E9CEECE01A339CEC3E1A8FA30" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_InterestExpenseDebtAbstract" xlink:label="loc_us-gaap_InterestExpenseDebtAbstract_500D6821F653C15185B6CEC3E1A81082" xlink:type="locator" />
    <link:definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_InterestExpenseDebtAbstract_500D6821F653C15185B6CEC3E1A81082" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_InterestExpenseDebtExcludingAmortization" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_D95A6566F3481F38327DCEC3E1A82703" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestExpenseDebtAbstract_500D6821F653C15185B6CEC3E1A81082" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_D95A6566F3481F38327DCEC3E1A82703" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AmortizationOfFinancingCostsAndDiscounts" xlink:label="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_199B92408FEE11487AF3CEC3E1A8469A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestExpenseDebtAbstract_500D6821F653C15185B6CEC3E1A81082" xlink:to="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_199B92408FEE11487AF3CEC3E1A8469A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_InterestExpenseDebt" xlink:label="loc_us-gaap_InterestExpenseDebt_778B4038BFF400DDFFC1CEC3E1A8882D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestExpenseDebtAbstract_500D6821F653C15185B6CEC3E1A81082" xlink:to="loc_us-gaap_InterestExpenseDebt_778B4038BFF400DDFFC1CEC3E1A8882D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract" xlink:label="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_58ECF46A8F737FD4AEF6CEC3E1A8FDBB" xlink:type="locator" />
    <link:definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_D0F6EAAF4B04B7449CDACEC3E19E5641" xlink:to="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_58ECF46A8F737FD4AEF6CEC3E1A8FDBB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PurchaseOfConvertibleNoteHedges" xlink:label="loc_regn_PurchaseOfConvertibleNoteHedges_89DF22C8DB476DAA6919CEC3E1A84339" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_58ECF46A8F737FD4AEF6CEC3E1A8FDBB" xlink:to="loc_regn_PurchaseOfConvertibleNoteHedges_89DF22C8DB476DAA6919CEC3E1A84339" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights_57185BF25F0BB6DB1FA9CEC3E1A8E684" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_58ECF46A8F737FD4AEF6CEC3E1A8FDBB" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights_57185BF25F0BB6DB1FA9CEC3E1A8E684" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromIssuanceOfWarrants" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfWarrants_44A900263F3D289ACDA7CEC3E1A88E57" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_58ECF46A8F737FD4AEF6CEC3E1A8FDBB" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfWarrants_44A900263F3D289ACDA7CEC3E1A88E57" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/DeferredRevenueDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementLineItems" xlink:label="loc_us-gaap_DeferredRevenueArrangementLineItems_006DEF3953F606C39F09CC08305E849C" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementByTypeTable" xlink:label="loc_us-gaap_DeferredRevenueArrangementByTypeTable_8EED45331FB1259E6762CC08305DC54B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_006DEF3953F606C39F09CC08305E849C" xlink:to="loc_us-gaap_DeferredRevenueArrangementByTypeTable_8EED45331FB1259E6762CC08305DC54B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeAxis" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeAxis_753E8575F8D7AFDE1639CC08305D1331" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DeferredRevenueArrangementByTypeTable_8EED45331FB1259E6762CC08305DC54B" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeAxis_753E8575F8D7AFDE1639CC08305D1331" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7E1B06B16BE8E2030B86CC08305D19DB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeAxis_753E8575F8D7AFDE1639CC08305D1331" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7E1B06B16BE8E2030B86CC08305D19DB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7E1B06B16BE8E2030B86CC08305D19DB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeAxis_753E8575F8D7AFDE1639CC08305D1331" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7E1B06B16BE8E2030B86CC08305D19DB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CollaborationAgreementWithSanofiMember" xlink:label="loc_regn_CollaborationAgreementWithSanofiMember_FA30AD61961D5E5D9205CC08305E30F6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7E1B06B16BE8E2030B86CC08305D19DB" xlink:to="loc_regn_CollaborationAgreementWithSanofiMember_FA30AD61961D5E5D9205CC08305E30F6" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CollaborationAgreementWithBayerHealthcareLlcMember" xlink:label="loc_regn_CollaborationAgreementWithBayerHealthcareLlcMember_02FE833256A3EA9D5C8DCC08305EBF5F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7E1B06B16BE8E2030B86CC08305D19DB" xlink:to="loc_regn_CollaborationAgreementWithBayerHealthcareLlcMember_02FE833256A3EA9D5C8DCC08305EBF5F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NonsoftwareLicenseArrangementMember" xlink:label="loc_us-gaap_NonsoftwareLicenseArrangementMember_ED4BFA8447188B066342CC08305E69AB" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7E1B06B16BE8E2030B86CC08305D19DB" xlink:to="loc_us-gaap_NonsoftwareLicenseArrangementMember_ED4BFA8447188B066342CC08305E69AB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_OtherDeferredRevenueAgreementsMember" xlink:label="loc_regn_OtherDeferredRevenueAgreementsMember_BD55BB8902E11DC9FEE6CC08305E4597" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7E1B06B16BE8E2030B86CC08305D19DB" xlink:to="loc_regn_OtherDeferredRevenueAgreementsMember_BD55BB8902E11DC9FEE6CC08305E4597" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueAndCreditsCurrentAbstract" xlink:label="loc_us-gaap_DeferredRevenueAndCreditsCurrentAbstract_4C037C5EF93CF367F723CC08305E687F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_006DEF3953F606C39F09CC08305E849C" xlink:to="loc_us-gaap_DeferredRevenueAndCreditsCurrentAbstract_4C037C5EF93CF367F723CC08305E687F" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueFromRelatedPartyCurrent" xlink:label="loc_regn_DeferredRevenueFromRelatedPartyCurrent_0B1C77563EF90F5FC347CC08305EE541" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueAndCreditsCurrentAbstract_4C037C5EF93CF367F723CC08305E687F" xlink:to="loc_regn_DeferredRevenueFromRelatedPartyCurrent_0B1C77563EF90F5FC347CC08305EE541" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueExcludingAmountsFromRelatedPartyCurrent" xlink:label="loc_regn_DeferredRevenueExcludingAmountsFromRelatedPartyCurrent_EC2A679EE42CAF780528CC08305E19E7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueAndCreditsCurrentAbstract_4C037C5EF93CF367F723CC08305E687F" xlink:to="loc_regn_DeferredRevenueExcludingAmountsFromRelatedPartyCurrent_EC2A679EE42CAF780528CC08305E19E7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueCurrent" xlink:label="loc_us-gaap_DeferredRevenueCurrent_71D576169EFB5EE2B4D7CC08305E68F7" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueAndCreditsCurrentAbstract_4C037C5EF93CF367F723CC08305E687F" xlink:to="loc_us-gaap_DeferredRevenueCurrent_71D576169EFB5EE2B4D7CC08305E68F7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueAndCreditsNoncurrentAbstract" xlink:label="loc_us-gaap_DeferredRevenueAndCreditsNoncurrentAbstract_367538859E14433ED9A7CC08305E945D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_006DEF3953F606C39F09CC08305E849C" xlink:to="loc_us-gaap_DeferredRevenueAndCreditsNoncurrentAbstract_367538859E14433ED9A7CC08305E945D" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueFromRelatedPartyNoncurrent" xlink:label="loc_regn_DeferredRevenueFromRelatedPartyNoncurrent_E6E70738926820221BB1CC08305E8032" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueAndCreditsNoncurrentAbstract_367538859E14433ED9A7CC08305E945D" xlink:to="loc_regn_DeferredRevenueFromRelatedPartyNoncurrent_E6E70738926820221BB1CC08305E8032" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DeferredRevenueExcludingAmountsFromRelatedPartyNonCurrent" xlink:label="loc_regn_DeferredRevenueExcludingAmountsFromRelatedPartyNonCurrent_3F2954FD5639E7451152CC08305E2359" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueAndCreditsNoncurrentAbstract_367538859E14433ED9A7CC08305E945D" xlink:to="loc_regn_DeferredRevenueExcludingAmountsFromRelatedPartyNonCurrent_3F2954FD5639E7451152CC08305E2359" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueNoncurrent" xlink:label="loc_us-gaap_DeferredRevenueNoncurrent_72DFF442D71C3BE9650ACC08305E76D1" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueAndCreditsNoncurrentAbstract_367538859E14433ED9A7CC08305E945D" xlink:to="loc_us-gaap_DeferredRevenueNoncurrent_72DFF442D71C3BE9650ACC08305E76D1" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/DocumentAndEntityInformation" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityInformationLineItems" xlink:label="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntitiesTable" xlink:label="loc_dei_EntitiesTable_B46E051910376855BF3D898D57E991F8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntitiesTable_B46E051910376855BF3D898D57E991F8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_F9C8CBE8DFF161558C21898D57E9A5D1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntitiesTable_B46E051910376855BF3D898D57E991F8" xlink:to="loc_us-gaap_StatementClassOfStockAxis_F9C8CBE8DFF161558C21898D57E9A5D1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_5435FA08BB9EDA7B8A34898D57EA5BAE_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_F9C8CBE8DFF161558C21898D57E9A5D1" xlink:to="loc_us-gaap_ClassOfStockDomain_5435FA08BB9EDA7B8A34898D57EA5BAE_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_5435FA08BB9EDA7B8A34898D57EA5BAE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_F9C8CBE8DFF161558C21898D57E9A5D1" xlink:to="loc_us-gaap_ClassOfStockDomain_5435FA08BB9EDA7B8A34898D57EA5BAE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_F2AF7F9BD97DB08283E3898D57EAAC42" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_5435FA08BB9EDA7B8A34898D57EA5BAE" xlink:to="loc_us-gaap_CommonStockMember_F2AF7F9BD97DB08283E3898D57EAAC42" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonClassAMember" xlink:label="loc_us-gaap_CommonClassAMember_7AF7B98727B0E22C4A4C898D57EA67C2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_5435FA08BB9EDA7B8A34898D57EA5BAE" xlink:to="loc_us-gaap_CommonClassAMember_7AF7B98727B0E22C4A4C898D57EA67C2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityRegistrantName" xlink:label="loc_dei_EntityRegistrantName_7220121CC9EC07B25B33898D57EAEE40" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntityRegistrantName_7220121CC9EC07B25B33898D57EAEE40" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityCentralIndexKey" xlink:label="loc_dei_EntityCentralIndexKey_8C9EBC8ADE823E0E93F0898D57EA0041" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntityCentralIndexKey_8C9EBC8ADE823E0E93F0898D57EA0041" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_CurrentFiscalYearEndDate" xlink:label="loc_dei_CurrentFiscalYearEndDate_F6787DA35B8A1740C6EE898D57EBBB87" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_CurrentFiscalYearEndDate_F6787DA35B8A1740C6EE898D57EBBB87" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityFilerCategory" xlink:label="loc_dei_EntityFilerCategory_CE771441529D1F38593B898D57EC1C8C" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntityFilerCategory_CE771441529D1F38593B898D57EC1C8C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_DocumentType" xlink:label="loc_dei_DocumentType_18F0633F75F8F693C04A898D57ECF013" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_DocumentType_18F0633F75F8F693C04A898D57ECF013" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_DocumentPeriodEndDate" xlink:label="loc_dei_DocumentPeriodEndDate_000358FED63045B14721898D57ECEC24" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_DocumentPeriodEndDate_000358FED63045B14721898D57ECEC24" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_DocumentFiscalYearFocus" xlink:label="loc_dei_DocumentFiscalYearFocus_3AD2539FDDCA18156946898D57ECB2D5" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_DocumentFiscalYearFocus_3AD2539FDDCA18156946898D57ECB2D5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="loc_dei_DocumentFiscalPeriodFocus_839AC1620A9F14DA9417898D57EC83C2" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_DocumentFiscalPeriodFocus_839AC1620A9F14DA9417898D57EC83C2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_AmendmentFlag" xlink:label="loc_dei_AmendmentFlag_53F22DF46996DD66CAF9898D57EC09AD" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_AmendmentFlag_53F22DF46996DD66CAF9898D57EC09AD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_89216DAC62571990241C898D57EC79C5" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_89216DAC62571990241C898D57EC79C5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityWellKnownSeasonedIssuer" xlink:label="loc_dei_EntityWellKnownSeasonedIssuer_025FC7A611DBD97B7F5C898D57ED83EF" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntityWellKnownSeasonedIssuer_025FC7A611DBD97B7F5C898D57ED83EF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityVoluntaryFilers" xlink:label="loc_dei_EntityVoluntaryFilers_B4ABB0815A120B946BD2898D57EDB28F" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntityVoluntaryFilers_B4ABB0815A120B946BD2898D57EDB28F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityCurrentReportingStatus" xlink:label="loc_dei_EntityCurrentReportingStatus_BD6D5BF67860D938E8D7898D57ED3CCA" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntityCurrentReportingStatus_BD6D5BF67860D938E8D7898D57ED3CCA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityPublicFloat" xlink:label="loc_dei_EntityPublicFloat_A3F822AD81395282A59D898D57ED9CB6" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_CE73D32575062FA5A5D3898D57EA4867" xlink:to="loc_dei_EntityPublicFloat_A3F822AD81395282A59D898D57ED9CB6" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/EmployeeSavingsPlanDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems_CF330805ECEE26DA362EBAF7FB82A6B4" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansDisclosuresTable" xlink:label="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansDisclosuresTable_0B6AE70E37746240082EBAF5D7579C8B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems_CF330805ECEE26DA362EBAF7FB82A6B4" xlink:to="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansDisclosuresTable_0B6AE70E37746240082EBAF5D7579C8B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6C5460E653176573EF0CBAF2B051E49E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansDisclosuresTable_0B6AE70E37746240082EBAF5D7579C8B" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6C5460E653176573EF0CBAF2B051E49E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1942AF20E519A08C3AE3BAF2B051672B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6C5460E653176573EF0CBAF2B051E49E" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1942AF20E519A08C3AE3BAF2B051672B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1942AF20E519A08C3AE3BAF2B051672B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6C5460E653176573EF0CBAF2B051E49E" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1942AF20E519A08C3AE3BAF2B051672B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_AAF758351168CE179027BAF2B051C4A4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansDisclosuresTable_0B6AE70E37746240082EBAF5D7579C8B" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_AAF758351168CE179027BAF2B051C4A4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PlanAssetCategoriesDomain" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_136B1A8DC546748BD64CBAF2B051F7ED_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_AAF758351168CE179027BAF2B051C4A4" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_136B1A8DC546748BD64CBAF2B051F7ED_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PlanAssetCategoriesDomain" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_136B1A8DC546748BD64CBAF2B051F7ED" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_AAF758351168CE179027BAF2B051C4A4" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_136B1A8DC546748BD64CBAF2B051F7ED" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis" xlink:label="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_58EB8E264CA1F5BB4142BAF2B051A6CD" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansDisclosuresTable_0B6AE70E37746240082EBAF5D7579C8B" xlink:to="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_58EB8E264CA1F5BB4142BAF2B051A6CD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_985A0E1CE1ABBD556F3FBAF2B051A76C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_58EB8E264CA1F5BB4142BAF2B051A6CD" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_985A0E1CE1ABBD556F3FBAF2B051A76C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_985A0E1CE1ABBD556F3FBAF2B051A76C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_58EB8E264CA1F5BB4142BAF2B051A6CD" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_985A0E1CE1ABBD556F3FBAF2B051A76C" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/ExecutiveStockPurchasePlanDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:label="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_6B43E96B4707F31711DD898D579C1884" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable_7FE0CB8562D02128FBE8898D579C75E0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_6B43E96B4707F31711DD898D579C1884" xlink:to="loc_us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable_7FE0CB8562D02128FBE8898D579C75E0" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis" xlink:label="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis_D38FA55DDB92249CA3C1898D579CC723" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable_7FE0CB8562D02128FBE8898D579C75E0" xlink:to="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis_D38FA55DDB92249CA3C1898D579CC723" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain" xlink:label="loc_us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain_C0E10C5DEEF8BF695D0F898D579C8DA2_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis_D38FA55DDB92249CA3C1898D579CC723" xlink:to="loc_us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain_C0E10C5DEEF8BF695D0F898D579C8DA2_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain" xlink:label="loc_us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain_C0E10C5DEEF8BF695D0F898D579C8DA2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis_D38FA55DDB92249CA3C1898D579CC723" xlink:to="loc_us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain_C0E10C5DEEF8BF695D0F898D579C8DA2" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ExecutiveStockPurchasePlanMember" xlink:label="loc_regn_ExecutiveStockPurchasePlanMember_896F8D70D9C95469172C898D579CF94E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain_C0E10C5DEEF8BF695D0F898D579C8DA2" xlink:to="loc_regn_ExecutiveStockPurchasePlanMember_896F8D70D9C95469172C898D579CF94E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP" xlink:label="loc_us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP_0E6908C56F650188E4D4898D579C903F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_6B43E96B4707F31711DD898D579C1884" xlink:to="loc_us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP_0E6908C56F650188E4D4898D579C903F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_24E9AF1664C6C7D89A91898D579C9A1C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_6B43E96B4707F31711DD898D579C1884" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_24E9AF1664C6C7D89A91898D579C9A1C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPNumberOfSuspenseShares" xlink:label="loc_us-gaap_EmployeeStockOwnershipPlanESOPNumberOfSuspenseShares_B0775871801EBD069ADE898D579C9F98" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_6B43E96B4707F31711DD898D579C1884" xlink:to="loc_us-gaap_EmployeeStockOwnershipPlanESOPNumberOfSuspenseShares_B0775871801EBD069ADE898D579C9F98" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/FairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CE10A58ED264F0A85A5ABCC0C1E36296" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_445C4FEBBEA6BAED302EBCC0C1D45509" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CE10A58ED264F0A85A5ABCC0C1E36296" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_445C4FEBBEA6BAED302EBCC0C1D45509" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_2017B5A68E3402FCC4D0BCC0C1D46F17" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_445C4FEBBEA6BAED302EBCC0C1D45509" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_2017B5A68E3402FCC4D0BCC0C1D46F17" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_E5144D4017FE80B6F3FEBCC0C1E37FD0_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_2017B5A68E3402FCC4D0BCC0C1D46F17" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_E5144D4017FE80B6F3FEBCC0C1E37FD0_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_E5144D4017FE80B6F3FEBCC0C1E37FD0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_2017B5A68E3402FCC4D0BCC0C1D46F17" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_E5144D4017FE80B6F3FEBCC0C1E37FD0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_6A2CAC7C25F6B2FF4739BCC0C1E323CD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_E5144D4017FE80B6F3FEBCC0C1E37FD0" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_6A2CAC7C25F6B2FF4739BCC0C1E323CD" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ClassificationOfInvestmentAxis" xlink:label="loc_regn_ClassificationOfInvestmentAxis_121351615B2CF83D13ECBCC0C1E34D9E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_445C4FEBBEA6BAED302EBCC0C1D45509" xlink:to="loc_regn_ClassificationOfInvestmentAxis_121351615B2CF83D13ECBCC0C1E34D9E" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ClassificationOfInvestmentDomain" xlink:label="loc_regn_ClassificationOfInvestmentDomain_F9CB8B43D9CAD281C0C9BCC0C1E3DDBB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_ClassificationOfInvestmentAxis_121351615B2CF83D13ECBCC0C1E34D9E" xlink:to="loc_regn_ClassificationOfInvestmentDomain_F9CB8B43D9CAD281C0C9BCC0C1E3DDBB_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ClassificationOfInvestmentDomain" xlink:label="loc_regn_ClassificationOfInvestmentDomain_F9CB8B43D9CAD281C0C9BCC0C1E3DDBB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_ClassificationOfInvestmentAxis_121351615B2CF83D13ECBCC0C1E34D9E" xlink:to="loc_regn_ClassificationOfInvestmentDomain_F9CB8B43D9CAD281C0C9BCC0C1E3DDBB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_UnrestrictedMember" xlink:label="loc_regn_UnrestrictedMember_3622C46D8DEADD242FB7BCC0C1E32230" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ClassificationOfInvestmentDomain_F9CB8B43D9CAD281C0C9BCC0C1E3DDBB" xlink:to="loc_regn_UnrestrictedMember_3622C46D8DEADD242FB7BCC0C1E32230" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RestrictedMember" xlink:label="loc_regn_RestrictedMember_12B975735299C6791F16BCC0C1E3E8C2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ClassificationOfInvestmentDomain_F9CB8B43D9CAD281C0C9BCC0C1E3DDBB" xlink:to="loc_regn_RestrictedMember_12B975735299C6791F16BCC0C1E3E8C2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_96171DF34513FAB87A39BCC0C1E38D65" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_445C4FEBBEA6BAED302EBCC0C1D45509" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_96171DF34513FAB87A39BCC0C1E38D65" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_32B4A377D47339D04EEEBCC0C1E3ED90_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_96171DF34513FAB87A39BCC0C1E38D65" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_32B4A377D47339D04EEEBCC0C1E3ED90_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_32B4A377D47339D04EEEBCC0C1E3ED90" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_96171DF34513FAB87A39BCC0C1E38D65" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_32B4A377D47339D04EEEBCC0C1E3ED90" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_5D71A83DC05E4585641BBCC0C1E349F9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_32B4A377D47339D04EEEBCC0C1E3ED90" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_5D71A83DC05E4585641BBCC0C1E349F9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_B0785C1308664B02852CBCC0C1E3A6E0" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_32B4A377D47339D04EEEBCC0C1E3ED90" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_B0785C1308664B02852CBCC0C1E3A6E0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_5DC1F68F79DCD9FD48B9BCC0C1E3B0B2" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_32B4A377D47339D04EEEBCC0C1E3ED90" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_5DC1F68F79DCD9FD48B9BCC0C1E3B0B2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_87E15C383E4D223670B5BCC0C1E3BBA0" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_32B4A377D47339D04EEEBCC0C1E3ED90" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_87E15C383E4D223670B5BCC0C1E3BBA0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_907BBC4FA49A09A1C7AFBCC0C1E38B49" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_445C4FEBBEA6BAED302EBCC0C1D45509" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_907BBC4FA49A09A1C7AFBCC0C1E38B49" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_247AF1B3D156CDFD41AFBCC0C1E36997_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_907BBC4FA49A09A1C7AFBCC0C1E38B49" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_247AF1B3D156CDFD41AFBCC0C1E36997_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_247AF1B3D156CDFD41AFBCC0C1E36997" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_907BBC4FA49A09A1C7AFBCC0C1E38B49" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_247AF1B3D156CDFD41AFBCC0C1E36997" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConvertibleDebtMember" xlink:label="loc_us-gaap_ConvertibleDebtMember_D55FE32D3CA3F8E569F4BCC0C1E37FAD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_247AF1B3D156CDFD41AFBCC0C1E36997" xlink:to="loc_us-gaap_ConvertibleDebtMember_D55FE32D3CA3F8E569F4BCC0C1E37FAD" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NumberOfMarketableSecuritiesHeld" xlink:label="loc_regn_NumberOfMarketableSecuritiesHeld_2C9E7C12AE71D2C3B1A1BCC0C1E3DC2E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CE10A58ED264F0A85A5ABCC0C1E36296" xlink:to="loc_regn_NumberOfMarketableSecuritiesHeld_2C9E7C12AE71D2C3B1A1BCC0C1E3DC2E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAbstract" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CE10A58ED264F0A85A5ABCC0C1E36296" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_USGovernmentObligationsFairValueDisclosure" xlink:label="loc_regn_USGovernmentObligationsFairValueDisclosure_6E49E9B8DFAE4B1BCFFABCC0C1E3E53F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:to="loc_regn_USGovernmentObligationsFairValueDisclosure_6E49E9B8DFAE4B1BCFFABCC0C1E3E53F" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_USGovernmentGuaranteedCorporateBondsFairValueDisclosure" xlink:label="loc_regn_USGovernmentGuaranteedCorporateBondsFairValueDisclosure_64948EE21AC4ACF2002ABCC0C1E3412E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:to="loc_regn_USGovernmentGuaranteedCorporateBondsFairValueDisclosure_64948EE21AC4ACF2002ABCC0C1E3412E" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_USGovernmentGuaranteedCollateralizedMortgageObligationsFairValueDisclosure" xlink:label="loc_regn_USGovernmentGuaranteedCollateralizedMortgageObligationsFairValueDisclosure_30ACF99B458F569D5A4FBCC0C1E3AD2D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:to="loc_regn_USGovernmentGuaranteedCollateralizedMortgageObligationsFairValueDisclosure_30ACF99B458F569D5A4FBCC0C1E3AD2D" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MunicipalBondsFairValueDisclosure" xlink:label="loc_regn_MunicipalBondsFairValueDisclosure_E84F2BCB94F2E98E89AFBCC0C1E3C073" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:to="loc_regn_MunicipalBondsFairValueDisclosure_E84F2BCB94F2E98E89AFBCC0C1E3C073" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MortgageBackedSecuritiesFairValueDisclosure" xlink:label="loc_regn_MortgageBackedSecuritiesFairValueDisclosure_5D80F8AEA5C1613A6917BCC0C1E39912" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:to="loc_regn_MortgageBackedSecuritiesFairValueDisclosure_5D80F8AEA5C1613A6917BCC0C1E39912" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_EquitySecurityFairValueDisclosure" xlink:label="loc_regn_EquitySecurityFairValueDisclosure_F545B3630AA2F9669B8CBCC0C1E382C0" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:to="loc_regn_EquitySecurityFairValueDisclosure_F545B3630AA2F9669B8CBCC0C1E382C0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesFairValueDisclosure" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesFairValueDisclosure_E8AECBFC2A89CB2DE5D0BCC0C1E3EE1B" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_C6E0C1D4F2DA31030365BCC0C1E32F09" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesFairValueDisclosure_E8AECBFC2A89CB2DE5D0BCC0C1E3EE1B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract" xlink:label="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_74CCD9C7D32AF4A74453BCC0C1E340A3" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CE10A58ED264F0A85A5ABCC0C1E36296" xlink:to="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_74CCD9C7D32AF4A74453BCC0C1E340A3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_8C7417093D31F028482EBCC0C1E31D5C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_74CCD9C7D32AF4A74453BCC0C1E340A3" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_8C7417093D31F028482EBCC0C1E31D5C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_B5D58A2A7186A1A0C622BCC0C1E3A19A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_74CCD9C7D32AF4A74453BCC0C1E340A3" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_B5D58A2A7186A1A0C622BCC0C1E3A19A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConvertibleDebtFairValueDisclosures" xlink:label="loc_us-gaap_ConvertibleDebtFairValueDisclosures_48311B4A990F7C3606D3BCC0C1E32DD7" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermConvertibleDebtCurrentAndNoncurrentAbstract_74CCD9C7D32AF4A74453BCC0C1E340A3" xlink:to="loc_us-gaap_ConvertibleDebtFairValueDisclosures_48311B4A990F7C3606D3BCC0C1E32DD7" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/IncomeTaxesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLossCarryforwardsLineItems" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_7D07CAC520D2474F8308D707B97D2215" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLossCarryforwardsTable" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_13430C14BE63C0F5E306D707B97D169D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_7D07CAC520D2474F8308D707B97D2215" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_13430C14BE63C0F5E306D707B97D169D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_2D1C731614355A445853D707B97D8A9B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_13430C14BE63C0F5E306D707B97D169D" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_2D1C731614355A445853D707B97D8A9B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_63D2597B203CA5B67EA6D707B97D9B6C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_2D1C731614355A445853D707B97D8A9B" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_63D2597B203CA5B67EA6D707B97D9B6C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_63D2597B203CA5B67EA6D707B97D9B6C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_2D1C731614355A445853D707B97D8A9B" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_63D2597B203CA5B67EA6D707B97D9B6C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_USFederalAndNewYorkStateMember" xlink:label="loc_regn_USFederalAndNewYorkStateMember_1260D8C71B91D717A569D707B97D9D5E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_63D2597B203CA5B67EA6D707B97D9B6C" xlink:to="loc_regn_USFederalAndNewYorkStateMember_1260D8C71B91D717A569D707B97D9D5E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:label="loc_us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_47735F797274951544ECD70D01EE907B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_7D07CAC520D2474F8308D707B97D2215" xlink:to="loc_us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_47735F797274951544ECD70D01EE907B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_83F8F08C2C40C11234EDD707B97D8E72" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_7D07CAC520D2474F8308D707B97D2215" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_83F8F08C2C40C11234EDD707B97D8E72" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLossCarryforwards" xlink:label="loc_us-gaap_OperatingLossCarryforwards_57EBD893AC8D60A82797D707B97EABE7" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_7D07CAC520D2474F8308D707B97D2215" xlink:to="loc_us-gaap_OperatingLossCarryforwards_57EBD893AC8D60A82797D707B97EABE7" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NetOperatingLossCarryForwardsExercisesOfNonqualifiedStockOptionsAndDisqualifyingDispositionsOfIncentiveStockOptions" xlink:label="loc_regn_NetOperatingLossCarryForwardsExercisesOfNonqualifiedStockOptionsAndDisqualifyingDispositionsOfIncentiveStockOptions_52785058BC638A8D934BD707B97E0B67" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_7D07CAC520D2474F8308D707B97D2215" xlink:to="loc_regn_NetOperatingLossCarryForwardsExercisesOfNonqualifiedStockOptionsAndDisqualifyingDispositionsOfIncentiveStockOptions_52785058BC638A8D934BD707B97E0B67" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/LegalMattersDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LossContingenciesTable" xlink:label="loc_us-gaap_LossContingenciesTable_CE7AA44871923C175F91A6DDAD3C6B81" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_us-gaap_LossContingenciesTable_CE7AA44871923C175F91A6DDAD3C6B81" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LitigationStatusAxis" xlink:label="loc_us-gaap_LitigationStatusAxis_F6762A26885DC0094039A748C5D513A9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_CE7AA44871923C175F91A6DDAD3C6B81" xlink:to="loc_us-gaap_LitigationStatusAxis_F6762A26885DC0094039A748C5D513A9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LitigationStatusDomain" xlink:label="loc_us-gaap_LitigationStatusDomain_937C367E9452ED095365A74AB8278BFE_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LitigationStatusAxis_F6762A26885DC0094039A748C5D513A9" xlink:to="loc_us-gaap_LitigationStatusDomain_937C367E9452ED095365A74AB8278BFE_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LitigationStatusDomain" xlink:label="loc_us-gaap_LitigationStatusDomain_937C367E9452ED095365A74AB8278BFE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LitigationStatusAxis_F6762A26885DC0094039A748C5D513A9" xlink:to="loc_us-gaap_LitigationStatusDomain_937C367E9452ED095365A74AB8278BFE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PendingLitigationMember" xlink:label="loc_us-gaap_PendingLitigationMember_672A3D2F9993DAE22BDFA74930E7950A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LitigationStatusDomain_937C367E9452ED095365A74AB8278BFE" xlink:to="loc_us-gaap_PendingLitigationMember_672A3D2F9993DAE22BDFA74930E7950A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LitigationCaseAxis" xlink:label="loc_us-gaap_LitigationCaseAxis_FA7ACD54BA4BCA051491A7539A6BA389" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_CE7AA44871923C175F91A6DDAD3C6B81" xlink:to="loc_us-gaap_LitigationCaseAxis_FA7ACD54BA4BCA051491A7539A6BA389" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LitigationCaseTypeDomain" xlink:label="loc_us-gaap_LitigationCaseTypeDomain_7975F027398A263AC77AA76330662DC2_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LitigationCaseAxis_FA7ACD54BA4BCA051491A7539A6BA389" xlink:to="loc_us-gaap_LitigationCaseTypeDomain_7975F027398A263AC77AA76330662DC2_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LitigationCaseTypeDomain" xlink:label="loc_us-gaap_LitigationCaseTypeDomain_7975F027398A263AC77AA76330662DC2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LitigationCaseAxis_FA7ACD54BA4BCA051491A7539A6BA389" xlink:to="loc_us-gaap_LitigationCaseTypeDomain_7975F027398A263AC77AA76330662DC2" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FirstDavisSmythCaseMember" xlink:label="loc_regn_FirstDavisSmythCaseMember_92E7211C37B5367DF34DA7541EFEE3B2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LitigationCaseTypeDomain_7975F027398A263AC77AA76330662DC2" xlink:to="loc_regn_FirstDavisSmythCaseMember_92E7211C37B5367DF34DA7541EFEE3B2" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LicenseAgreementWithGenentechMember" xlink:label="loc_regn_LicenseAgreementWithGenentechMember_11E73563B10366836F7CA7689CDE0988" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LitigationCaseTypeDomain_7975F027398A263AC77AA76330662DC2" xlink:to="loc_regn_LicenseAgreementWithGenentechMember_11E73563B10366836F7CA7689CDE0988" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_SecondDavisSmythCaseMember" xlink:label="loc_regn_SecondDavisSmythCaseMember_A1FF356F1DB09C21B937A75C32EDE838" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LitigationCaseTypeDomain_7975F027398A263AC77AA76330662DC2" xlink:to="loc_regn_SecondDavisSmythCaseMember_A1FF356F1DB09C21B937A75C32EDE838" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LossContingenciesByGeographicAreaAxis" xlink:label="loc_regn_LossContingenciesByGeographicAreaAxis_641F284CA56BD067887FA6DDAD3D4DB6" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_CE7AA44871923C175F91A6DDAD3C6B81" xlink:to="loc_regn_LossContingenciesByGeographicAreaAxis_641F284CA56BD067887FA6DDAD3D4DB6" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LossContingenciesByGeographicAreaDomain" xlink:label="loc_regn_LossContingenciesByGeographicAreaDomain_E6A4AD6C40D56397C356A6DDAD3D039C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_LossContingenciesByGeographicAreaAxis_641F284CA56BD067887FA6DDAD3D4DB6" xlink:to="loc_regn_LossContingenciesByGeographicAreaDomain_E6A4AD6C40D56397C356A6DDAD3D039C_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LossContingenciesByGeographicAreaDomain" xlink:label="loc_regn_LossContingenciesByGeographicAreaDomain_E6A4AD6C40D56397C356A6DDAD3D039C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_LossContingenciesByGeographicAreaAxis_641F284CA56BD067887FA6DDAD3D4DB6" xlink:to="loc_regn_LossContingenciesByGeographicAreaDomain_E6A4AD6C40D56397C356A6DDAD3D039C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/country/2012/country-2012-01-31.xsd#country_US" xlink:label="loc_country_US_C2228AEE4C48ED0CCA0EA6DDAD3DD6EE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_LossContingenciesByGeographicAreaDomain_E6A4AD6C40D56397C356A6DDAD3D039C" xlink:to="loc_country_US_C2228AEE4C48ED0CCA0EA6DDAD3DD6EE" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_EuropeMember" xlink:label="loc_regn_EuropeMember_47812027BB55A220FE83A6DDAD3D8E0D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_LossContingenciesByGeographicAreaDomain_E6A4AD6C40D56397C356A6DDAD3D039C" xlink:to="loc_regn_EuropeMember_47812027BB55A220FE83A6DDAD3D8E0D" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LossContingencyNumberOfLitigationCases" xlink:label="loc_regn_LossContingencyNumberOfLitigationCases_596EDFB65CD259EFB1D8A765A2A4FE8C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_regn_LossContingencyNumberOfLitigationCases_596EDFB65CD259EFB1D8A765A2A4FE8C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LossContingencyNewClaimsFiledNumber" xlink:label="loc_us-gaap_LossContingencyNewClaimsFiledNumber_8C90752893DDA626BD96A752180732BD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_us-gaap_LossContingencyNewClaimsFiledNumber_8C90752893DDA626BD96A752180732BD" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AmountOfPaymentMadeUponReachingMinimumSalesTarget" xlink:label="loc_regn_AmountOfPaymentMadeUponReachingMinimumSalesTarget_2923B765AB3EA135E744A6DDAD3D3FD6" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_regn_AmountOfPaymentMadeUponReachingMinimumSalesTarget_2923B765AB3EA135E744A6DDAD3D3FD6" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CumulativeSalesRangeMinimum" xlink:label="loc_regn_CumulativeSalesRangeMinimum_96F77D08CD119D889D32A6DDAD3D9E87" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_regn_CumulativeSalesRangeMinimum_96F77D08CD119D889D32A6DDAD3D9E87" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RoyaltyPaymentPercentageOfCumulativeSalesWithinRange" xlink:label="loc_regn_RoyaltyPaymentPercentageOfCumulativeSalesWithinRange_AD2954E0EFC3C82DA784A6DDAD3D6A16" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_regn_RoyaltyPaymentPercentageOfCumulativeSalesWithinRange_AD2954E0EFC3C82DA784A6DDAD3D6A16" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CumulativeSalesRangeMaximum" xlink:label="loc_regn_CumulativeSalesRangeMaximum_631881000A3B5B70E7A3A6DDAD3D3797" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_regn_CumulativeSalesRangeMaximum_631881000A3B5B70E7A3A6DDAD3D3797" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RoyaltyPaymentPercentageOfCumulativeSalesOverRange" xlink:label="loc_regn_RoyaltyPaymentPercentageOfCumulativeSalesOverRange_986B199D84809C2985E8A6DDAD3DC5C4" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_regn_RoyaltyPaymentPercentageOfCumulativeSalesOverRange_986B199D84809C2985E8A6DDAD3DC5C4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LossContingencyPendingClaimsNumber" xlink:label="loc_us-gaap_LossContingencyPendingClaimsNumber_7CFE2D5841DDA550384EA6DDAD3D66EF" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_D1303DB8DD5F85DDF1DBA6DDAD3D07D8" xlink:to="loc_us-gaap_LossContingencyPendingClaimsNumber_7CFE2D5841DDA550384EA6DDAD3D66EF" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/LongTermIncentivePlansDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5F5A38E4C76C3402A8A8CF9D33C32C52" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5F5A38E4C76C3402A8A8CF9D33C32C52" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_344E53A80FCE3C916D6ACF9D33C39014" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5F5A38E4C76C3402A8A8CF9D33C32C52" xlink:to="loc_us-gaap_PlanNameAxis_344E53A80FCE3C916D6ACF9D33C39014" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_F4D9D741956462311E74CF9D33C35DDB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_344E53A80FCE3C916D6ACF9D33C39014" xlink:to="loc_us-gaap_PlanNameDomain_F4D9D741956462311E74CF9D33C35DDB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_F4D9D741956462311E74CF9D33C35DDB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_344E53A80FCE3C916D6ACF9D33C39014" xlink:to="loc_us-gaap_PlanNameDomain_F4D9D741956462311E74CF9D33C35DDB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LongTermIncentivePlan2000Member" xlink:label="loc_regn_LongTermIncentivePlan2000Member_C9929447EB0AD896C8A9CF9D33C30728" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_F4D9D741956462311E74CF9D33C35DDB" xlink:to="loc_regn_LongTermIncentivePlan2000Member_C9929447EB0AD896C8A9CF9D33C30728" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_C594E86952FA4D1F8998CF9D33C32DB4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5F5A38E4C76C3402A8A8CF9D33C32C52" xlink:to="loc_us-gaap_AwardTypeAxis_C594E86952FA4D1F8998CF9D33C32DB4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_C594E86952FA4D1F8998CF9D33C32DB4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_C594E86952FA4D1F8998CF9D33C32DB4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockOptionsMember" xlink:label="loc_us-gaap_StockOptionsMember_AC54F0CBF35F315BADDFCF9D33C3602F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394" xlink:to="loc_us-gaap_StockOptionsMember_AC54F0CBF35F315BADDFCF9D33C3602F" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PhantomStockMember" xlink:label="loc_regn_PhantomStockMember_C2C2C3074ED60801B554CF9D33C308D7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394" xlink:to="loc_regn_PhantomStockMember_C2C2C3074ED60801B554CF9D33C308D7" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NonPerformanceBasedStockOptionsMember" xlink:label="loc_regn_NonPerformanceBasedStockOptionsMember_3730965E4E91DF6FC055CF9D33C37E2C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394" xlink:to="loc_regn_NonPerformanceBasedStockOptionsMember_3730965E4E91DF6FC055CF9D33C37E2C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PerformanceBasedStockOptionsMember" xlink:label="loc_regn_PerformanceBasedStockOptionsMember_8FB04B6B4BF6FE976085CF9D33C3CE78" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394" xlink:to="loc_regn_PerformanceBasedStockOptionsMember_8FB04B6B4BF6FE976085CF9D33C3CE78" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RestrictedStockMember" xlink:label="loc_us-gaap_RestrictedStockMember_DE58DA8959BEF768820DCF9D33C3EB76" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C2B41A8EA22A284A5A1DCF9D33C32394" xlink:to="loc_us-gaap_RestrictedStockMember_DE58DA8959BEF768820DCF9D33C3EB76" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_2C5E5988229280EB0747CF9D33C367E9" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5F5A38E4C76C3402A8A8CF9D33C32C52" xlink:to="loc_us-gaap_RangeAxis_2C5E5988229280EB0747CF9D33C367E9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_E5EB09322FCD4D2221F7CF9D33C383B8_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_2C5E5988229280EB0747CF9D33C367E9" xlink:to="loc_us-gaap_RangeMember_E5EB09322FCD4D2221F7CF9D33C383B8_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_E5EB09322FCD4D2221F7CF9D33C383B8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_2C5E5988229280EB0747CF9D33C367E9" xlink:to="loc_us-gaap_RangeMember_E5EB09322FCD4D2221F7CF9D33C383B8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_87B2B59B2E850DC7E5F6CF9D33C30533" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_E5EB09322FCD4D2221F7CF9D33C383B8" xlink:to="loc_us-gaap_MinimumMember_87B2B59B2E850DC7E5F6CF9D33C30533" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_EB08170591FB842BF9DACF9D33C3FD91" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_E5EB09322FCD4D2221F7CF9D33C383B8" xlink:to="loc_us-gaap_MaximumMember_EB08170591FB842BF9DACF9D33C3FD91" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_33B58BC5A9BBB65882EECF9D33C3B2B8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_33B58BC5A9BBB65882EECF9D33C3B2B8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_54D3F1DAC6E72F2FC67ACF9D33C365F1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_54D3F1DAC6E72F2FC67ACF9D33C365F1" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpiration" xlink:label="loc_regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpiration_930C3715A49B9BF48AFECF9D33C3C92A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpiration_930C3715A49B9BF48AFECF9D33C3C92A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodFromVestingDateAwardsMayBeReceived" xlink:label="loc_regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodFromVestingDateAwardsMayBeReceived_B7DCCFC233A931010D3DCF9D33C3ACE2" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodFromVestingDateAwardsMayBeReceived_B7DCCFC233A931010D3DCF9D33C3ACE2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_ADDA572A9CA53887C52DCF9D33C32E49" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_ADDA572A9CA53887C52DCF9D33C32E49" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_8E4608D22A6AD0DBF427CF9D33C3674B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_8E4608D22A6AD0DBF427CF9D33C3674B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_2FA9A57A0BFFB3BFCBAFCF9D33C33576" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_8E4608D22A6AD0DBF427CF9D33C3674B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_2FA9A57A0BFFB3BFCBAFCF9D33C33576" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_3AFDA36F5FA02929E75DCF9D33C3AB2B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_8E4608D22A6AD0DBF427CF9D33C3674B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_3AFDA36F5FA02929E75DCF9D33C3AB2B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_BA124CCBF82B2F2A4E6ECF9D33C30409" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_8E4608D22A6AD0DBF427CF9D33C3674B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_BA124CCBF82B2F2A4E6ECF9D33C30409" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_03B001472A74B4F7516BCF9D33C3D05D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_8E4608D22A6AD0DBF427CF9D33C3674B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_03B001472A74B4F7516BCF9D33C3D05D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_8BAE93D88C2E59A088C1CF9D33C39A7A" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_8E4608D22A6AD0DBF427CF9D33C3674B" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_8BAE93D88C2E59A088C1CF9D33C39A7A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_31E7522505AD5ED4EEB9CF9D33C35743" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_31E7522505AD5ED4EEB9CF9D33C35743" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_FD61C5FF7B1473BCADCECF9D33C34B46" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_FD61C5FF7B1473BCADCECF9D33C34B46" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_C73AD76512E17E36DEF0CF9D33C3BB2F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_C73AD76512E17E36DEF0CF9D33C3BB2F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_73E87704172856E8E80ECF9D33C358BE" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_73E87704172856E8E80ECF9D33C358BE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_991EC79CC575CC370348CF9D33C3C3CC" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_991EC79CC575CC370348CF9D33C3C3CC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_07315A09B3BDFEDEC2A9CF9D33C3581D" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_07315A09B3BDFEDEC2A9CF9D33C3581D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_FB9A8840C28C1852780DCF9D33C36CEF" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_FB9A8840C28C1852780DCF9D33C36CEF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_2DA5900BB8835BFB3719CF9D33C33518" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_2DA5900BB8835BFB3719CF9D33C33518" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_0002A70F6F933C4F429ECF9D33C3FF57" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_0002A70F6F933C4F429ECF9D33C3FF57" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_CA0D20265F2F899DF783CF9D33C3226F" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_CA0D20265F2F899DF783CF9D33C3226F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_6965B0CE539E8A1DC202CF9D33C3E2A1" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_6965B0CE539E8A1DC202CF9D33C3E2A1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_6F1DA134F80A2BB33BB6CF9D33C3201E" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_6F1DA134F80A2BB33BB6CF9D33C3201E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_848643F466A4B4F38970CF9D33C3CC8B" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_848643F466A4B4F38970CF9D33C3CC8B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_0B5DBBDFB984F96F6241CF9D33C36007" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_0B5DBBDFB984F96F6241CF9D33C36007" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_D0CC5532F57E0720AD5ECF9D33C3BF56" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_D0CC5532F57E0720AD5ECF9D33C3BF56" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_E34866EDC51CC841E7D1CF9D33C3560E" xlink:type="locator" />
    <link:definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_B2964410898F7166FC19CF9D33C3E3E2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_E34866EDC51CC841E7D1CF9D33C3560E" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ExercisePriceEqualToMarketPriceAbstract" xlink:label="loc_regn_ExercisePriceEqualToMarketPriceAbstract_106A392F299B4E2BFA24CF9D33C3C8C6" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_regn_ExercisePriceEqualToMarketPriceAbstract_106A392F299B4E2BFA24CF9D33C3C8C6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_6BFDC10317577417762DCF9D33C32AC2" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ExercisePriceEqualToMarketPriceAbstract_106A392F299B4E2BFA24CF9D33C3C8C6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_6BFDC10317577417762DCF9D33C32AC2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_2BB3987C0D602465A3DCCF9D33C35787" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_2BB3987C0D602465A3DCCF9D33C35787" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_1A02722933C3BD9081E7CF9D33C3EE47" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_2BB3987C0D602465A3DCCF9D33C35787" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_1A02722933C3BD9081E7CF9D33C3EE47" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_FF7603F36ED10DB0B19ACF9D33C34986" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_2BB3987C0D602465A3DCCF9D33C35787" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_FF7603F36ED10DB0B19ACF9D33C34986" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_51661F8E3D7E1BAE625FCF9D33C3A93A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_2BB3987C0D602465A3DCCF9D33C35787" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_51661F8E3D7E1BAE625FCF9D33C3A93A" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedNumber" xlink:label="loc_regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedNumber_E4B3B2EAAE208706D031CF9D33C3B9F8" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_2BB3987C0D602465A3DCCF9D33C35787" xlink:to="loc_regn_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedNumber_E4B3B2EAAE208706D031CF9D33C3B9F8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_80839DEB4EA0E7D69B41CF9D33C3F0C6" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_80839DEB4EA0E7D69B41CF9D33C3F0C6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_729F7980C356A1E3E09ACF9D33C38CA6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_80839DEB4EA0E7D69B41CF9D33C3F0C6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_729F7980C356A1E3E09ACF9D33C38CA6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_4965CBF4922A372EE003CF9D33CD413E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_80839DEB4EA0E7D69B41CF9D33C3F0C6" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_4965CBF4922A372EE003CF9D33CD413E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_A165C3651E262D5EF9B5CF9D33CD66CC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_80839DEB4EA0E7D69B41CF9D33C3F0C6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_A165C3651E262D5EF9B5CF9D33CD66CC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_F0516A2DE16DCEEF2FABCF9D33CDFDA2" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_80839DEB4EA0E7D69B41CF9D33C3F0C6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_F0516A2DE16DCEEF2FABCF9D33CDFDA2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_92C5FF3E0CA5E52460E8CF9D33CD47BD" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_92C5FF3E0CA5E52460E8CF9D33CD47BD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_579D6BDE9478F392A94FCF9D33CD45EB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_92C5FF3E0CA5E52460E8CF9D33CD47BD" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_579D6BDE9478F392A94FCF9D33CD45EB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_95339B0F615BCA7C39CCCF9D33CD67CF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_92C5FF3E0CA5E52460E8CF9D33CD47BD" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_95339B0F615BCA7C39CCCF9D33CD67CF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_17263FBBEA4ED8F45848CF9D33CD3475" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_92C5FF3E0CA5E52460E8CF9D33CD47BD" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_17263FBBEA4ED8F45848CF9D33CD3475" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_BDB7D1A28025F246F43BCF9D33CD64A8" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DAF698BFCC55AB185BACCF9D33C3AA3A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_BDB7D1A28025F246F43BCF9D33CD64A8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_90F1BA0ABA1A2D25A8E9CF9D33CD8FD6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_BDB7D1A28025F246F43BCF9D33CD64A8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_90F1BA0ABA1A2D25A8E9CF9D33CD8FD6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_06FB1676B52A793CDB8FCF9D33CDBB24" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_BDB7D1A28025F246F43BCF9D33CD64A8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_06FB1676B52A793CDB8FCF9D33CDBB24" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_74880013B51872CFDFAACF9D33CD60C8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_BDB7D1A28025F246F43BCF9D33CD64A8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_74880013B51872CFDFAACF9D33CD60C8" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/MarketableSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_5724B4505DB2E1C7C732AC1FD8525D04" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:to="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_5724B4505DB2E1C7C732AC1FD8525D04" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_5F6C8A705605BBE9E692AC1FD85233B9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_5724B4505DB2E1C7C732AC1FD8525D04" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_5F6C8A705605BBE9E692AC1FD85233B9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_059EC8334E15757EC0EDAC1FD8531ABF_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_5F6C8A705605BBE9E692AC1FD85233B9" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_059EC8334E15757EC0EDAC1FD8531ABF_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_059EC8334E15757EC0EDAC1FD8531ABF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_5F6C8A705605BBE9E692AC1FD85233B9" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_059EC8334E15757EC0EDAC1FD8531ABF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtSecuritiesMember" xlink:label="loc_us-gaap_DebtSecuritiesMember_66F742A7981B6C048B57AC1FD8537F50" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_059EC8334E15757EC0EDAC1FD8531ABF" xlink:to="loc_us-gaap_DebtSecuritiesMember_66F742A7981B6C048B57AC1FD8537F50" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_04D08CBCD76BB0A3A075AC1FD853A6B1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesMember_66F742A7981B6C048B57AC1FD8537F50" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_04D08CBCD76BB0A3A075AC1FD853A6B1" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_USGovernmentGuaranteedCorporateBondsMember" xlink:label="loc_regn_USGovernmentGuaranteedCorporateBondsMember_527846A04D871D149E87AC1FD853A685" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesMember_66F742A7981B6C048B57AC1FD8537F50" xlink:to="loc_regn_USGovernmentGuaranteedCorporateBondsMember_527846A04D871D149E87AC1FD853A685" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_D0F7DE1D27E7B94346D0AC1FD853D9D9" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesMember_66F742A7981B6C048B57AC1FD8537F50" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_D0F7DE1D27E7B94346D0AC1FD853D9D9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MunicipalBondsMember" xlink:label="loc_us-gaap_MunicipalBondsMember_1053569A33441A060FE0AC1FD853D2BE" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesMember_66F742A7981B6C048B57AC1FD8537F50" xlink:to="loc_us-gaap_MunicipalBondsMember_1053569A33441A060FE0AC1FD853D2BE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember_D7E98BB97AB22220A29EAC1FD853A34D" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesMember_66F742A7981B6C048B57AC1FD8537F50" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember_D7E98BB97AB22220A29EAC1FD853A34D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EquitySecuritiesMember" xlink:label="loc_us-gaap_EquitySecuritiesMember_7A5F3C04F4C1435EA695AC1FD8532143" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_059EC8334E15757EC0EDAC1FD8531ABF" xlink:to="loc_us-gaap_EquitySecuritiesMember_7A5F3C04F4C1435EA695AC1FD8532143" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ClassificationOfInvestmentAxis" xlink:label="loc_regn_ClassificationOfInvestmentAxis_EC4D7B14DA417E74C2C7AC1FD8533314" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_5724B4505DB2E1C7C732AC1FD8525D04" xlink:to="loc_regn_ClassificationOfInvestmentAxis_EC4D7B14DA417E74C2C7AC1FD8533314" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ClassificationOfInvestmentDomain" xlink:label="loc_regn_ClassificationOfInvestmentDomain_6912DF88BD7E68E9FF65AC1FD8535D7D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_regn_ClassificationOfInvestmentAxis_EC4D7B14DA417E74C2C7AC1FD8533314" xlink:to="loc_regn_ClassificationOfInvestmentDomain_6912DF88BD7E68E9FF65AC1FD8535D7D_default" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ClassificationOfInvestmentDomain" xlink:label="loc_regn_ClassificationOfInvestmentDomain_6912DF88BD7E68E9FF65AC1FD8535D7D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_regn_ClassificationOfInvestmentAxis_EC4D7B14DA417E74C2C7AC1FD8533314" xlink:to="loc_regn_ClassificationOfInvestmentDomain_6912DF88BD7E68E9FF65AC1FD8535D7D" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_UnrestrictedMember" xlink:label="loc_regn_UnrestrictedMember_7512C7EC9CDFD12D4A57AC1FD853232D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ClassificationOfInvestmentDomain_6912DF88BD7E68E9FF65AC1FD8535D7D" xlink:to="loc_regn_UnrestrictedMember_7512C7EC9CDFD12D4A57AC1FD853232D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RestrictedStockMember" xlink:label="loc_us-gaap_RestrictedStockMember_10BB3957E3A226602534AC1FD8536938" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ClassificationOfInvestmentDomain_6912DF88BD7E68E9FF65AC1FD8535D7D" xlink:to="loc_us-gaap_RestrictedStockMember_10BB3957E3A226602534AC1FD8536938" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RestrictedMember" xlink:label="loc_regn_RestrictedMember_6E3D623386D15D137F9FAC1FD853C0B5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_ClassificationOfInvestmentDomain_6912DF88BD7E68E9FF65AC1FD8535D7D" xlink:to="loc_regn_RestrictedMember_6E3D623386D15D137F9FAC1FD853C0B5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAmortizedCost" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAmortizedCost_882C5330A4F71436298FAC1FD8534CE8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAmortizedCost_882C5330A4F71436298FAC1FD8534CE8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGain" xlink:label="loc_us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGain_3B2D477C11A1A04271F1AC1FD854DEAB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:to="loc_us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGain_3B2D477C11A1A04271F1AC1FD854DEAB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLosses1" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLosses1_354779CBB2311E4BDF96AC1FD8541FEE" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLosses1_354779CBB2311E4BDF96AC1FD8541FEE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesFairValueDisclosure" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesFairValueDisclosure_95A98EA7F88880D288EAAC1FD854B751" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesFairValueDisclosure_95A98EA7F88880D288EAAC1FD854B751" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_DD5D192ABF3BE4540765AC1FD854BBE1" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_DD5D192ABF3BE4540765AC1FD854BBE1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_FBC10B311B58A59DD9A6AC1FD854930E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_DD5D192ABF3BE4540765AC1FD854BBE1" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_FBC10B311B58A59DD9A6AC1FD854930E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_8EEC0A67782675C39431AC1FD8542950" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_DD5D192ABF3BE4540765AC1FD854BBE1" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_8EEC0A67782675C39431AC1FD8542950" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_20D15A71AE5BDEA163E7AC1FD854ECF6" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_DD5D192ABF3BE4540765AC1FD854BBE1" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_20D15A71AE5BDEA163E7AC1FD854ECF6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_F1D4A21D27C61C768A71AC1FD854091B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_DD5D192ABF3BE4540765AC1FD854BBE1" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_F1D4A21D27C61C768A71AC1FD854091B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueAbstract" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueAbstract_4E644526FEF309FF529CAC1FD8541B7C" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueAbstract_4E644526FEF309FF529CAC1FD8541B7C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue_185B3E0CCB58A28D4C20AC1FD854B756" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueAbstract_4E644526FEF309FF529CAC1FD8541B7C" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue_185B3E0CCB58A28D4C20AC1FD854B756" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue_35AEF186115AAE050ED3AC1FD8543FB5" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueAbstract_4E644526FEF309FF529CAC1FD8541B7C" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue_35AEF186115AAE050ED3AC1FD8543FB5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue_41AE53A1BF4E161C4BACAC1FD85419EA" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueAbstract_4E644526FEF309FF529CAC1FD8541B7C" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue_41AE53A1BF4E161C4BACAC1FD85419EA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAbstract" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAbstract_ED02704E83D434CC30C4AC1FD8549E31" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_D8927B94FBF7BB343E2DAC1FD85353D8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAbstract_ED02704E83D434CC30C4AC1FD8549E31" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses2" xlink:label="loc_us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses2_782AA274BAF36BFCADA9AC1FD85403B4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAbstract_ED02704E83D434CC30C4AC1FD8549E31" xlink:to="loc_us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses2_782AA274BAF36BFCADA9AC1FD85403B4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses2" xlink:label="loc_us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses2_E5133A3C9B8390FB235AAC1FD854AFC3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAbstract_ED02704E83D434CC30C4AC1FD8549E31" xlink:to="loc_us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses2_E5133A3C9B8390FB235AAC1FD854AFC3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses2" xlink:label="loc_us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses2_5D42FCA8116915D2AA06AC1FD854B36B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAbstract_ED02704E83D434CC30C4AC1FD8549E31" xlink:to="loc_us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses2_5D42FCA8116915D2AA06AC1FD854B36B" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/NetIncomeLossPerShareDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_A65D22BFD7964E0C8B73898D5873AC90" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_5038F5F4F17C90917411898D587399A6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_A65D22BFD7964E0C8B73898D5873AC90" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_5038F5F4F17C90917411898D587399A6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_F415DE41EB0C4392EE27898D5873F1E3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_5038F5F4F17C90917411898D587399A6" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_F415DE41EB0C4392EE27898D5873F1E3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_71CCEFCFE1FB9737365A898D5873B690_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_F415DE41EB0C4392EE27898D5873F1E3" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_71CCEFCFE1FB9737365A898D5873B690_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_71CCEFCFE1FB9737365A898D5873B690" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_F415DE41EB0C4392EE27898D5873F1E3" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_71CCEFCFE1FB9737365A898D5873B690" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockOptionsMember" xlink:label="loc_us-gaap_StockOptionsMember_D61B00D178A202A56891898D5873B7AD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_71CCEFCFE1FB9737365A898D5873B690" xlink:to="loc_us-gaap_StockOptionsMember_D61B00D178A202A56891898D5873B7AD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RestrictedStockMember" xlink:label="loc_us-gaap_RestrictedStockMember_86A547E23225877D52AD898D587356EE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_71CCEFCFE1FB9737365A898D5873B690" xlink:to="loc_us-gaap_RestrictedStockMember_86A547E23225877D52AD898D587356EE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConvertibleDebtSecuritiesMember" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_922BAC7E795C6081E704898D587317EA" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_71CCEFCFE1FB9737365A898D5873B690" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_922BAC7E795C6081E704898D587317EA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_WarrantMember" xlink:label="loc_us-gaap_WarrantMember_228DE1EF9287C119005A898D587370AA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_71CCEFCFE1FB9737365A898D5873B690" xlink:to="loc_us-gaap_WarrantMember_228DE1EF9287C119005A898D587370AA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_653811C9900F26578D08898D5873D9C6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_A65D22BFD7964E0C8B73898D5873AC90" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_653811C9900F26578D08898D5873D9C6" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/NetProductSalesDetails" xlink:type="extended">
    <link:loc xlink:href="regn-20121231.xsd#regn_RevenueDisclosureLineItems" xlink:label="loc_regn_RevenueDisclosureLineItems_9499CF7DF64D5CA92A74898D582C1C08" xlink:type="locator" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RevenueDisclosureTable" xlink:label="loc_regn_RevenueDisclosureTable_61D544F944DA12455D20898D582B4B57" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_regn_RevenueDisclosureLineItems_9499CF7DF64D5CA92A74898D582C1C08" xlink:to="loc_regn_RevenueDisclosureTable_61D544F944DA12455D20898D582B4B57" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_DD6A8E9E808D77AA18E5898D582BA2E6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_regn_RevenueDisclosureTable_61D544F944DA12455D20898D582B4B57" xlink:to="loc_us-gaap_ProductOrServiceAxis_DD6A8E9E808D77AA18E5898D582BA2E6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_21DC0EF9CC2917A11BFF898D582B7EC2_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_DD6A8E9E808D77AA18E5898D582BA2E6" xlink:to="loc_us-gaap_ProductsAndServicesDomain_21DC0EF9CC2917A11BFF898D582B7EC2_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_21DC0EF9CC2917A11BFF898D582B7EC2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_DD6A8E9E808D77AA18E5898D582BA2E6" xlink:to="loc_us-gaap_ProductsAndServicesDomain_21DC0EF9CC2917A11BFF898D582B7EC2" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_EyleaNetProductSalesMember" xlink:label="loc_regn_EyleaNetProductSalesMember_4A1ECBCCAE41D3FFACBE898D582BC3D2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_21DC0EF9CC2917A11BFF898D582B7EC2" xlink:to="loc_regn_EyleaNetProductSalesMember_4A1ECBCCAE41D3FFACBE898D582BC3D2" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ArcalystNetProductSalesMember" xlink:label="loc_regn_ArcalystNetProductSalesMember_FDA973E1373009342B63898D582B6D51" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_21DC0EF9CC2917A11BFF898D582B7EC2" xlink:to="loc_regn_ArcalystNetProductSalesMember_FDA973E1373009342B63898D582B6D51" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_D43E8F51F1EF1AE84E77898D582BABD5" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_regn_RevenueDisclosureTable_61D544F944DA12455D20898D582B4B57" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_D43E8F51F1EF1AE84E77898D582BABD5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_1CB62C0A7FB2E9A18EE9898D582B4F4E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_D43E8F51F1EF1AE84E77898D582BABD5" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_1CB62C0A7FB2E9A18EE9898D582B4F4E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_1CB62C0A7FB2E9A18EE9898D582B4F4E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_D43E8F51F1EF1AE84E77898D582BABD5" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_1CB62C0A7FB2E9A18EE9898D582B4F4E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CustomerConcentrationRiskMember" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_375B30CA7BACF7704172898D582B34E8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_1CB62C0A7FB2E9A18EE9898D582B4F4E" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_375B30CA7BACF7704172898D582B34E8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_6F8B1273BF62E0286B43898D582B978F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_regn_RevenueDisclosureTable_61D544F944DA12455D20898D582B4B57" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_6F8B1273BF62E0286B43898D582B978F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_65AB66D5756AB3548DC9898D582B5AC0_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_6F8B1273BF62E0286B43898D582B978F" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_65AB66D5756AB3548DC9898D582B5AC0_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_65AB66D5756AB3548DC9898D582B5AC0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_6F8B1273BF62E0286B43898D582B978F" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_65AB66D5756AB3548DC9898D582B5AC0" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_GrosssalesrevenueMember" xlink:label="loc_regn_GrosssalesrevenueMember_D43F976D98E9B4337245898D582B1124" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_65AB66D5756AB3548DC9898D582B5AC0" xlink:to="loc_regn_GrosssalesrevenueMember_D43F976D98E9B4337245898D582B1124" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ValuationAllowancesAndReservesTypeAxis" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_75F7FFA881F0F5744DE1898D582C47CB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_regn_RevenueDisclosureTable_61D544F944DA12455D20898D582B4B57" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_75F7FFA881F0F5744DE1898D582C47CB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ValuationAllowancesAndReservesDomain" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_FA8144BB90AB3888315D898D582C49E5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_75F7FFA881F0F5744DE1898D582C47CB" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_FA8144BB90AB3888315D898D582C49E5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ValuationAllowancesAndReservesDomain" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_FA8144BB90AB3888315D898D582C49E5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_75F7FFA881F0F5744DE1898D582C47CB" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_FA8144BB90AB3888315D898D582C49E5" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RebatesAndChargebacksMember" xlink:label="loc_regn_RebatesAndChargebacksMember_3B4D5262FBE0CD0FD4D4898D582C9DE5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_FA8144BB90AB3888315D898D582C49E5" xlink:to="loc_regn_RebatesAndChargebacksMember_3B4D5262FBE0CD0FD4D4898D582C9DE5" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DistributionRelatedFeesMember" xlink:label="loc_regn_DistributionRelatedFeesMember_39E4CA904FF801C8406B898D582CB883" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_FA8144BB90AB3888315D898D582C49E5" xlink:to="loc_regn_DistributionRelatedFeesMember_39E4CA904FF801C8406B898D582CB883" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_OtherSalesRelatedDeductionsMember" xlink:label="loc_regn_OtherSalesRelatedDeductionsMember_5DDCA01C84092967AC8C898D582CCC09" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_FA8144BB90AB3888315D898D582C49E5" xlink:to="loc_regn_OtherSalesRelatedDeductionsMember_5DDCA01C84092967AC8C898D582CCC09" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_2568F07B11CE17B5FD39898D582CD107" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_RevenueDisclosureLineItems_9499CF7DF64D5CA92A74898D582C1C08" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_2568F07B11CE17B5FD39898D582CD107" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_SalesRevenueProductLineExcludingDeferredSalesRecognized" xlink:label="loc_regn_SalesRevenueProductLineExcludingDeferredSalesRecognized_456DF0E65F54826539BB898D582CF060" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_RevenueDisclosureLineItems_9499CF7DF64D5CA92A74898D582C1C08" xlink:to="loc_regn_SalesRevenueProductLineExcludingDeferredSalesRecognized_456DF0E65F54826539BB898D582CF060" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RecognitionOfDeferredRevenue" xlink:label="loc_us-gaap_RecognitionOfDeferredRevenue_11A99D98A7AF50FDE666898D582CCB03" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_RevenueDisclosureLineItems_9499CF7DF64D5CA92A74898D582C1C08" xlink:to="loc_us-gaap_RecognitionOfDeferredRevenue_11A99D98A7AF50FDE666898D582CCB03" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ChangeInAccountingEstimateDeferredRevenueLowerNetLossPerShare" xlink:label="loc_regn_ChangeInAccountingEstimateDeferredRevenueLowerNetLossPerShare_12CB896B1F43F578E973898D582CAA28" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_RevenueDisclosureLineItems_9499CF7DF64D5CA92A74898D582C1C08" xlink:to="loc_regn_ChangeInAccountingEstimateDeferredRevenueLowerNetLossPerShare_12CB896B1F43F578E973898D582CAA28" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RisksAndUncertaintiesAbstract" xlink:label="loc_us-gaap_RisksAndUncertaintiesAbstract_F6D43FFD416BAADCB944898D582CAE8F" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_RevenueDisclosureLineItems_9499CF7DF64D5CA92A74898D582C1C08" xlink:to="loc_us-gaap_RisksAndUncertaintiesAbstract_F6D43FFD416BAADCB944898D582CAE8F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerPercentage" xlink:label="loc_us-gaap_EntityWideRevenueMajorCustomerPercentage_3D0E02722CBA1C93921F898D582CA79A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RisksAndUncertaintiesAbstract_F6D43FFD416BAADCB944898D582CAE8F" xlink:to="loc_us-gaap_EntityWideRevenueMajorCustomerPercentage_3D0E02722CBA1C93921F898D582CA79A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MovementInValuationAllowancesAndReservesRollForward" xlink:label="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_86A8D0E1AC7293E48465898D582CA249" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_regn_RevenueDisclosureLineItems_9499CF7DF64D5CA92A74898D582C1C08" xlink:to="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_86A8D0E1AC7293E48465898D582CA249" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ValuationAllowancesAndReservesBalance" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesBalance_AC7358E28FEA5E2C6FF1898D582CDC68" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_86A8D0E1AC7293E48465898D582CA249" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesBalance_AC7358E28FEA5E2C6FF1898D582CDC68" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ValuationAllowancesAndReservesChargedToOtherAccounts" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesChargedToOtherAccounts_6EA1E9340877368A6C23898D582DC4A0" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_86A8D0E1AC7293E48465898D582CA249" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesChargedToOtherAccounts_6EA1E9340877368A6C23898D582DC4A0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ValuationAllowancesAndReservesDeductions" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDeductions_82C08249D970E49C6469898D582D5B34" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MovementInValuationAllowancesAndReservesRollForward_86A8D0E1AC7293E48465898D582CA249" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDeductions_82C08249D970E49C6469898D582D5B34" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/PropertyPlantAndEquipmentDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_AC18DB46F8369E30D4EAB58EAEB15399" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_AC18DB46F8369E30D4EAB58EAEB15399" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_69FF3C574B466876CE73B58EAEB15FB3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_AC18DB46F8369E30D4EAB58EAEB15399" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_69FF3C574B466876CE73B58EAEB15FB3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_69FF3C574B466876CE73B58EAEB15FB3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_69FF3C574B466876CE73B58EAEB15FB3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LandMember" xlink:label="loc_us-gaap_LandMember_5CB9B9B5BE157E7D892DB58EAEB21EA2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:to="loc_us-gaap_LandMember_5CB9B9B5BE157E7D892DB58EAEB21EA2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_BuildingAndBuildingImprovementsMember" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_A06B908B1D58CB435664B58EAEB2C0F5" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_A06B908B1D58CB435664B58EAEB2C0F5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_95620C4BAEFEBB6D230FB58EAEB2AD39" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_95620C4BAEFEBB6D230FB58EAEB2AD39" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConstructionInProgressMember" xlink:label="loc_us-gaap_ConstructionInProgressMember_F88CC289FB94BA956C70B58EAEB26385" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:to="loc_us-gaap_ConstructionInProgressMember_F88CC289FB94BA956C70B58EAEB26385" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EquipmentMember" xlink:label="loc_us-gaap_EquipmentMember_78C9E3FE0E34480796BDB58EAEB2EE62" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:to="loc_us-gaap_EquipmentMember_78C9E3FE0E34480796BDB58EAEB2EE62" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FurnitureComputerAndOfficeEquipmentAndOtherMember" xlink:label="loc_regn_FurnitureComputerAndOfficeEquipmentAndOtherMember_ED517B2763CC9AC05E8AB58EAEB239F3" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:to="loc_regn_FurnitureComputerAndOfficeEquipmentAndOtherMember_ED517B2763CC9AC05E8AB58EAEB239F3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AssetsHeldUnderCapitalLeasesMember" xlink:label="loc_us-gaap_AssetsHeldUnderCapitalLeasesMember_33C308B41520A0F46A7BB58EAEB2D801" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:to="loc_us-gaap_AssetsHeldUnderCapitalLeasesMember_33C308B41520A0F46A7BB58EAEB2D801" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NewLaboratoryAndOfficeFacilitiesMember" xlink:label="loc_regn_NewLaboratoryAndOfficeFacilitiesMember_C87245B77998854354C6B58EAEB20A8F" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F5368EB07141BE6BCB2AB58EAEB2954A" xlink:to="loc_regn_NewLaboratoryAndOfficeFacilitiesMember_C87245B77998854354C6B58EAEB20A8F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ChangeInAccountingEstimateByTypeAxis" xlink:label="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_A0BB76A9C412FEEFDF46B58EAEB21EBC" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_AC18DB46F8369E30D4EAB58EAEB15399" xlink:to="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_A0BB76A9C412FEEFDF46B58EAEB21EBC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ChangeInAccountingEstimateTypeDomain" xlink:label="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_B2786B1788882B3798E3B58EAEB23A00_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_A0BB76A9C412FEEFDF46B58EAEB21EBC" xlink:to="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_B2786B1788882B3798E3B58EAEB23A00_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ChangeInAccountingEstimateTypeDomain" xlink:label="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_B2786B1788882B3798E3B58EAEB23A00" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_A0BB76A9C412FEEFDF46B58EAEB21EBC" xlink:to="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_B2786B1788882B3798E3B58EAEB23A00" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ServiceLifeMember" xlink:label="loc_us-gaap_ServiceLifeMember_50A5D128CD217FFF9198B58EAEB241D6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_B2786B1788882B3798E3B58EAEB23A00" xlink:to="loc_us-gaap_ServiceLifeMember_50A5D128CD217FFF9198B58EAEB241D6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_E43256BDE29D4EC71584B58EAEB23612" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_E43256BDE29D4EC71584B58EAEB23612" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_DE09EF71DFEC95D69EF8B58EAEB2C9A0" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_DE09EF71DFEC95D69EF8B58EAEB2C9A0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_CB346B96617AD52708BEB58EAEB29E40" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_CB346B96617AD52708BEB58EAEB29E40" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_E17F88DC02457BC4060FB58EAEB314DD" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_E17F88DC02457BC4060FB58EAEB314DD" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ChangeInAccountingEstimateDecreaseInDepreciationExpense" xlink:label="loc_regn_ChangeInAccountingEstimateDecreaseInDepreciationExpense_6CB6DC57197CA5DD6858B58EAEB30534" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_regn_ChangeInAccountingEstimateDecreaseInDepreciationExpense_6CB6DC57197CA5DD6858B58EAEB30534" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_ChangeInAccountingEstimateDecreaseInNetLossPerShare" xlink:label="loc_regn_ChangeInAccountingEstimateDecreaseInNetLossPerShare_5DB6413C122A7F80E7C1B58EAEB35DBD" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_regn_ChangeInAccountingEstimateDecreaseInNetLossPerShare_5DB6413C122A7F80E7C1B58EAEB35DBD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_A6C57A328E26FA7965C3B58EAEB3C9BC" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_A6C57A328E26FA7965C3B58EAEB3C9BC" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CostsIncurredByLandlordToConstructNewLaboratoryAndOfficeFacilities" xlink:label="loc_regn_CostsIncurredByLandlordToConstructNewLaboratoryAndOfficeFacilities_B277ECDE350CE2F53939B58EAEB3D598" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_641CDF750CB474A4BEE0B58EAEB24B10" xlink:to="loc_regn_CostsIncurredByLandlordToConstructNewLaboratoryAndOfficeFacilities_B277ECDE350CE2F53939B58EAEB3D598" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/StockholdersEquityDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_E69D95899F80B8FD113ED4041AE129E5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_E69D95899F80B8FD113ED4041AE129E5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_551DBF6768B51A75D6DAD4041AE1A2A0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_E69D95899F80B8FD113ED4041AE129E5" xlink:to="loc_us-gaap_StatementClassOfStockAxis_551DBF6768B51A75D6DAD4041AE1A2A0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_13E7213E815425564745D4041AE165AD_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_551DBF6768B51A75D6DAD4041AE1A2A0" xlink:to="loc_us-gaap_ClassOfStockDomain_13E7213E815425564745D4041AE165AD_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_13E7213E815425564745D4041AE165AD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_551DBF6768B51A75D6DAD4041AE1A2A0" xlink:to="loc_us-gaap_ClassOfStockDomain_13E7213E815425564745D4041AE165AD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonClassAMember" xlink:label="loc_us-gaap_CommonClassAMember_165695968571283EE589D4041AE1B5F9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_13E7213E815425564745D4041AE165AD" xlink:to="loc_us-gaap_CommonClassAMember_165695968571283EE589D4041AE1B5F9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_8A1DF7D459C0D37EC632D4041AE134A6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_13E7213E815425564745D4041AE165AD" xlink:to="loc_us-gaap_CommonStockMember_8A1DF7D459C0D37EC632D4041AE134A6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_0E71FD814D1E8ACE1AF5D4041AE1253A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_E69D95899F80B8FD113ED4041AE129E5" xlink:to="loc_dei_LegalEntityAxis_0E71FD814D1E8ACE1AF5D4041AE1253A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_95ABC47E8BBD57C95863D4041AE1932E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_0E71FD814D1E8ACE1AF5D4041AE1253A" xlink:to="loc_dei_EntityDomain_95ABC47E8BBD57C95863D4041AE1932E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_95ABC47E8BBD57C95863D4041AE1932E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_0E71FD814D1E8ACE1AF5D4041AE1253A" xlink:to="loc_dei_EntityDomain_95ABC47E8BBD57C95863D4041AE1932E" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_SanofiMember" xlink:label="loc_regn_SanofiMember_D5B461571D8A04F4517BD4041AE12B2D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_95ABC47E8BBD57C95863D4041AE1932E" xlink:to="loc_regn_SanofiMember_D5B461571D8A04F4517BD4041AE12B2D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_883C816D054161302587D4041AE1A971" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_E69D95899F80B8FD113ED4041AE129E5" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_883C816D054161302587D4041AE1A971" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8D91CE4A4841F2AA6985D4041AE19FA3_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_883C816D054161302587D4041AE1A971" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8D91CE4A4841F2AA6985D4041AE19FA3_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8D91CE4A4841F2AA6985D4041AE19FA3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_883C816D054161302587D4041AE1A971" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8D91CE4A4841F2AA6985D4041AE19FA3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConvertibleDebtMember" xlink:label="loc_us-gaap_ConvertibleDebtMember_2160FD91D68008B90336D4041AE1F723" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8D91CE4A4841F2AA6985D4041AE19FA3" xlink:to="loc_us-gaap_ConvertibleDebtMember_2160FD91D68008B90336D4041AE1F723" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_077E772BD7493187E4E4D4041AE1D5BF" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_E69D95899F80B8FD113ED4041AE129E5" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_077E772BD7493187E4E4D4041AE1D5BF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_77E5A293395E37FF71B1D4041AE1FF13_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_077E772BD7493187E4E4D4041AE1D5BF" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_77E5A293395E37FF71B1D4041AE1FF13_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_77E5A293395E37FF71B1D4041AE1FF13" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_077E772BD7493187E4E4D4041AE1D5BF" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_77E5A293395E37FF71B1D4041AE1FF13" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_WarrantTransactionsMember" xlink:label="loc_regn_WarrantTransactionsMember_3AA850476448134325E2D4041AE1B5A8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_77E5A293395E37FF71B1D4041AE1FF13" xlink:to="loc_regn_WarrantTransactionsMember_3AA850476448134325E2D4041AE1B5A8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeAxis" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_DE90FEF6A840C515E94BD4041AE15381" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_E69D95899F80B8FD113ED4041AE129E5" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_DE90FEF6A840C515E94BD4041AE15381" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeDomain" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_189F2B1B56CDF76DFA5CD4041AE17577_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_DE90FEF6A840C515E94BD4041AE15381" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_189F2B1B56CDF76DFA5CD4041AE17577_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OptionIndexedToIssuersEquityTypeDomain" xlink:label="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_189F2B1B56CDF76DFA5CD4041AE17577" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeAxis_DE90FEF6A840C515E94BD4041AE15381" xlink:to="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_189F2B1B56CDF76DFA5CD4041AE17577" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CallOptionMember" xlink:label="loc_us-gaap_CallOptionMember_6B7CA419DC265C7442A9D4041AE19438" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OptionIndexedToIssuersEquityTypeDomain_189F2B1B56CDF76DFA5CD4041AE17577" xlink:to="loc_us-gaap_CallOptionMember_6B7CA419DC265C7442A9D4041AE19438" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_6669562B3BFBB6295326D4041AE169E5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_6669562B3BFBB6295326D4041AE169E5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_96E57C6FC1EFC54545A5D4041AE1A0CB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_96E57C6FC1EFC54545A5D4041AE1A0CB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockVotingRights" xlink:label="loc_us-gaap_CommonStockVotingRights_4D01E02E666369150685D4041AE1DD6D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_CommonStockVotingRights_4D01E02E666369150685D4041AE1DD6D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_D11835E7036E40077B52D4041AE14207" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_D11835E7036E40077B52D4041AE14207" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NumberOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased" xlink:label="loc_regn_NumberOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased_1DFAB5B4338B59057219D4041AE16613" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_regn_NumberOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased_1DFAB5B4338B59057219D4041AE16613" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_CashPriceOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased" xlink:label="loc_regn_CashPriceOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased_C6744C75FB40637182F2D4041AE14495" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_regn_CashPriceOfNewlyIssuedUnregisteredSharesOfCommonStockPurchased_C6744C75FB40637182F2D4041AE14495" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_DescriptionOfPeriodOfStandstillProvisions" xlink:label="loc_regn_DescriptionOfPeriodOfStandstillProvisions_6DCCA6D38AF70D906EFCD4041AE1B629" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_regn_DescriptionOfPeriodOfStandstillProvisions_6DCCA6D38AF70D906EFCD4041AE1B629" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_MaximumPercentageOfOutstandingSharesThatMayBeAcquiredUnderStandstillProvisions" xlink:label="loc_regn_MaximumPercentageOfOutstandingSharesThatMayBeAcquiredUnderStandstillProvisions_3FB70504A50C5FE3F778D4041AE1D6CB" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_regn_MaximumPercentageOfOutstandingSharesThatMayBeAcquiredUnderStandstillProvisions_3FB70504A50C5FE3F778D4041AE1D6CB" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PercentageOfOutstandingSharesOfCommonStockRelatedToProhibitionOnSellingToBeneficialOwners" xlink:label="loc_regn_PercentageOfOutstandingSharesOfCommonStockRelatedToProhibitionOnSellingToBeneficialOwners_73E1C2A7C4A14B756029D4041AE1F610" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_regn_PercentageOfOutstandingSharesOfCommonStockRelatedToProhibitionOnSellingToBeneficialOwners_73E1C2A7C4A14B756029D4041AE1F610" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PercentageOfOutstandingSharesOrVotingRightsUnderAmendedInvestorAgreement" xlink:label="loc_regn_PercentageOfOutstandingSharesOrVotingRightsUnderAmendedInvestorAgreement_266D9E28EDA45B37B5B8D4041AE151B0" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_regn_PercentageOfOutstandingSharesOrVotingRightsUnderAmendedInvestorAgreement_266D9E28EDA45B37B5B8D4041AE151B0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_5706BCAC20B49427B52ED4041AE15607" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_5706BCAC20B49427B52ED4041AE15607" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_808F891EA86C93745470D4041AE1F448" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_808F891EA86C93745470D4041AE1F448" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_6763BC65D9F378409263D4041AE123B6" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_6763BC65D9F378409263D4041AE123B6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebt" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebt_C6111554E5438F72263CD4041AE16EB6" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebt_C6111554E5438F72263CD4041AE16EB6" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PurchaseOfConvertibleNoteHedges" xlink:label="loc_regn_PurchaseOfConvertibleNoteHedges_19D4618738A004134AD7D4041AE13802" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_regn_PurchaseOfConvertibleNoteHedges_19D4618738A004134AD7D4041AE13802" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights_34BF11C2C6756D3A40DDD4041AE1FCCE" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights_34BF11C2C6756D3A40DDD4041AE1FCCE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromIssuanceOfWarrants" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfWarrants_0917671F015DE720619CD4041AE16880" xlink:type="locator" />
    <link:definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F6D0F1DC1C0F04C521CED4041AE15363" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfWarrants_0917671F015DE720619CD4041AE16880" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/SummaryOfSignificantAccountingPoliciesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_595D408FAC499F367561CA802FF19999" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_6193EB79070263313C0DCA802FF15332" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_595D408FAC499F367561CA802FF19999" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_6193EB79070263313C0DCA802FF15332" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_EA5FC1FCABB210C2BE46CA802FF1C101" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_6193EB79070263313C0DCA802FF15332" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_EA5FC1FCABB210C2BE46CA802FF1C101" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9007EC5D6149B73CDEABCA802FF118F4_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_EA5FC1FCABB210C2BE46CA802FF1C101" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9007EC5D6149B73CDEABCA802FF118F4_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9007EC5D6149B73CDEABCA802FF118F4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_EA5FC1FCABB210C2BE46CA802FF1C101" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9007EC5D6149B73CDEABCA802FF118F4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_BuildingAndBuildingImprovementsMember" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_9AD2FF9BB50B43FC03FDCA802FF1A08A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9007EC5D6149B73CDEABCA802FF118F4" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_9AD2FF9BB50B43FC03FDCA802FF1A08A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EquipmentMember" xlink:label="loc_us-gaap_EquipmentMember_A87EBC8EA02A958F2460CA802FF18D3C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9007EC5D6149B73CDEABCA802FF118F4" xlink:to="loc_us-gaap_EquipmentMember_A87EBC8EA02A958F2460CA802FF18D3C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_B59D636625B2B53F3FF5CA802FF12455" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9007EC5D6149B73CDEABCA802FF118F4" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_B59D636625B2B53F3FF5CA802FF12455" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_68F812EA282C2CFA31B1CA802FF11F03" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_6193EB79070263313C0DCA802FF15332" xlink:to="loc_us-gaap_RangeAxis_68F812EA282C2CFA31B1CA802FF11F03" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_F2FA2445B83D82C1E307CA802FF11C8E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_68F812EA282C2CFA31B1CA802FF11F03" xlink:to="loc_us-gaap_RangeMember_F2FA2445B83D82C1E307CA802FF11C8E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_F2FA2445B83D82C1E307CA802FF11C8E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_68F812EA282C2CFA31B1CA802FF11F03" xlink:to="loc_us-gaap_RangeMember_F2FA2445B83D82C1E307CA802FF11C8E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_8FBF1188D1DF3BA65AB6CA802FF1C634" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_F2FA2445B83D82C1E307CA802FF11C8E" xlink:to="loc_us-gaap_MinimumMember_8FBF1188D1DF3BA65AB6CA802FF1C634" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_66206920F7DB7C4CF904CA802FF1DE07" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_F2FA2445B83D82C1E307CA802FF11C8E" xlink:to="loc_us-gaap_MaximumMember_66206920F7DB7C4CF904CA802FF1DE07" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_DED957407C2B7BDE9A47CA802FF2BF09" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_595D408FAC499F367561CA802FF19999" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_DED957407C2B7BDE9A47CA802FF2BF09" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.regeneron.com/role/TechnologyLicensingAgreementsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementLineItems" xlink:label="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementByTypeTable" xlink:label="loc_us-gaap_DeferredRevenueArrangementByTypeTable_1660208E44BBDCFD1223898D5819622C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_us-gaap_DeferredRevenueArrangementByTypeTable_1660208E44BBDCFD1223898D5819622C" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeAxis" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeAxis_8E17C993A33DAF218955898D58195B44" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DeferredRevenueArrangementByTypeTable_1660208E44BBDCFD1223898D5819622C" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeAxis_8E17C993A33DAF218955898D58195B44" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7AF0AAE4D29A3AAAAABD898D5819E246_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeAxis_8E17C993A33DAF218955898D58195B44" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7AF0AAE4D29A3AAAAABD898D5819E246_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7AF0AAE4D29A3AAAAABD898D5819E246" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeAxis_8E17C993A33DAF218955898D58195B44" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7AF0AAE4D29A3AAAAABD898D5819E246" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NonsoftwareLicenseArrangementMember" xlink:label="loc_us-gaap_NonsoftwareLicenseArrangementMember_D8276506CE2E0AC57712898D5819839C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeDomain_7AF0AAE4D29A3AAAAABD898D5819E246" xlink:to="loc_us-gaap_NonsoftwareLicenseArrangementMember_D8276506CE2E0AC57712898D5819839C" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LicenseAgreementWithAstellasPharmaIncMember" xlink:label="loc_regn_LicenseAgreementWithAstellasPharmaIncMember_073C0D4ABA4C9B50801C898D5819D663" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonsoftwareLicenseArrangementMember_D8276506CE2E0AC57712898D5819839C" xlink:to="loc_regn_LicenseAgreementWithAstellasPharmaIncMember_073C0D4ABA4C9B50801C898D5819D663" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_LicenseAgreementWithAstrazenecaUkLimitedMember" xlink:label="loc_regn_LicenseAgreementWithAstrazenecaUkLimitedMember_153D879C840BCB701669898D581A9607" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonsoftwareLicenseArrangementMember_D8276506CE2E0AC57712898D5819839C" xlink:to="loc_regn_LicenseAgreementWithAstrazenecaUkLimitedMember_153D879C840BCB701669898D581A9607" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NonExclusiveLicenseAgreementTerm" xlink:label="loc_regn_NonExclusiveLicenseAgreementTerm_AB195AFABEF71FA06C57898D581A61AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_regn_NonExclusiveLicenseAgreementTerm_AB195AFABEF71FA06C57898D581A61AE" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AnnualNonRefundableLicensingPaymentReceived" xlink:label="loc_regn_AnnualNonRefundableLicensingPaymentReceived_BD3058F53F311968312A898D581A0A59" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_regn_AnnualNonRefundableLicensingPaymentReceived_BD3058F53F311968312A898D581A0A59" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_AdvancePaymentReceivedDueToAmendedLicensingAgreement" xlink:label="loc_regn_AdvancePaymentReceivedDueToAmendedLicensingAgreement_706994F35AD3B0B4CCC7898D581A6F6E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_regn_AdvancePaymentReceivedDueToAmendedLicensingAgreement_706994F35AD3B0B4CCC7898D581A6F6E" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_RecognitionPeriodForAdvancePayment" xlink:label="loc_regn_RecognitionPeriodForAdvancePayment_49150E93872D0F74505B898D581A456B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_regn_RecognitionPeriodForAdvancePayment_49150E93872D0F74505B898D581A456B" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_FuturePaymentToBeMadeUnderTermsOfAmendedLicensingAgreement" xlink:label="loc_regn_FuturePaymentToBeMadeUnderTermsOfAmendedLicensingAgreement_6C866BA7284A06CAC447898D581A73C9" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_regn_FuturePaymentToBeMadeUnderTermsOfAmendedLicensingAgreement_6C866BA7284A06CAC447898D581A73C9" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NumberOfDaysOfAdvanceWrittenNoticeToTerminateAgreement" xlink:label="loc_regn_NumberOfDaysOfAdvanceWrittenNoticeToTerminateAgreement_EE1528FB7F706B0FFE87898D581A400B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_regn_NumberOfDaysOfAdvanceWrittenNoticeToTerminateAgreement_EE1528FB7F706B0FFE87898D581A400B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LicensesRevenue" xlink:label="loc_us-gaap_LicensesRevenue_731B7D67AC06EBC6C634898D581A5656" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_us-gaap_LicensesRevenue_731B7D67AC06EBC6C634898D581A5656" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_PeriodOverWhichAnnualPaymentIsRecognizedAsRevenue" xlink:label="loc_regn_PeriodOverWhichAnnualPaymentIsRecognizedAsRevenue_2F37B27741C13C86E465898D581A21D8" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_regn_PeriodOverWhichAnnualPaymentIsRecognizedAsRevenue_2F37B27741C13C86E465898D581A21D8" xlink:type="arc" />
    <link:loc xlink:href="regn-20121231.xsd#regn_NumberOfAnnualPaymentsNotReceivedDueToCancellationOfObligation" xlink:label="loc_regn_NumberOfAnnualPaymentsNotReceivedDueToCancellationOfObligation_26770410A8BDE7582C56898D581A2E56" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_regn_NumberOfAnnualPaymentsNotReceivedDueToCancellationOfObligation_26770410A8BDE7582C56898D581A2E56" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenue" xlink:label="loc_us-gaap_DeferredRevenue_4909D9F0B07CA7224D59898D581ABAFC" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_536BB3DD68522CE2CF5B898D581A0CF0" xlink:to="loc_us-gaap_DeferredRevenue_4909D9F0B07CA7224D59898D581ABAFC" xlink:type="arc" />
  </link:definitionLink>
</link:linkbase>
