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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-03-01T19:45:39.9662690+00:00 -->
<!-- Version            : 5.0.1.571 -->
<!-- Package ID         : c9eeee61fa69408b833f9f3abaa0e21a -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="fulc_SeriesBStockPurchaseAgreementMember" name="SeriesBStockPurchaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_GrossProceedsThresholdOfAutomaticConversionOfPreferredStockToCommonStock" name="GrossProceedsThresholdOfAutomaticConversionOfPreferredStockToCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="fulc_NumberOfDirectorsStockholdersAreEntitledToElect" name="NumberOfDirectorsStockholdersAreEntitledToElect" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_PreferredStockConversionPricePerShare" name="PreferredStockConversionPricePerShare" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="fulc_MinimumPercentageOfShareholdersVoteOrWrittenConsentRequiredForAutomaticConversion" name="MinimumPercentageOfShareholdersVoteOrWrittenConsentRequiredForAutomaticConversion" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_MinimumPercentageOfOutstandingSharesOfPreferredStockForLiquidationPreference" name="MinimumPercentageOfOutstandingSharesOfPreferredStockForLiquidationPreference" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_NumberOfCommonStockVotingRights" name="NumberOfCommonStockVotingRights" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="fulc_SharesReservedForConversionOfPreferredStockAndFutureIssuanceOfCommonStock" name="SharesReservedForConversionOfPreferredStockAndFutureIssuanceOfCommonStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="fulc_GrantsUnderTwoThousandNineteenStockIncentivePlanMember" name="GrantsUnderTwoThousandNineteenStockIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_GrantsUnderTwoThousandNineteenEmployeeStockPurchasePlanMember" name="GrantsUnderTwoThousandNineteenEmployeeStockPurchasePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_TwoThousandSixteenStockIncentivePlanMember" name="TwoThousandSixteenStockIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_TwoThousandNineteenStockIncentivePlanMember" name="TwoThousandNineteenStockIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_IncreaseInCommonStockCapitalSharesReservedForFutureIssuance" name="IncreaseInCommonStockCapitalSharesReservedForFutureIssuance" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_IncreaseInNumberOfSharesReservedForIssuancePercentOfCommonStockOutstanding" name="IncreaseInNumberOfSharesReservedForIssuancePercentOfCommonStockOutstanding" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRepurchasedInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRepurchasedInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRepurchasedInPeriodWeightedAverageGrantDateFairValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRepurchasedInPeriodWeightedAverageGrantDateFairValue" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_ResearchDevelopmentAndSalesArrangementAggregateAmountReceivableUponAchievementOfMilestone" name="ResearchDevelopmentAndSalesArrangementAggregateAmountReceivableUponAchievementOfMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_RangeOfTieredRoyaltiesOnNetSales" name="RangeOfTieredRoyaltiesOnNetSales" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_NonRefundableUpfrontPaymentsReceived" name="NonRefundableUpfrontPaymentsReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_ResearchMilestoneMember" name="ResearchMilestoneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_ClinicalAndRegulatoryMilestonesMember" name="ClinicalAndRegulatoryMilestonesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_WorldwideNetSalesMilestonesMember" name="WorldwideNetSalesMilestonesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_MyoKardiaCollaborationAgreementMember" name="MyoKardiaCollaborationAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_DeferredRevenueCurrentAndNoncurrent" name="DeferredRevenueCurrentAndNoncurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_ReimbursementPaymentRecieved" name="ReimbursementPaymentRecieved" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_UnbilledAccountsReceivableRelatedToReimbursableResearchAndDevelopmentCost" name="UnbilledAccountsReceivableRelatedToReimbursableResearchAndDevelopmentCost" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_CumulativeCatchUpAdjustment" name="CumulativeCatchUpAdjustment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_UpfrontPayment" name="UpfrontPayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_PaymentForPrepaidResearchFunding" name="PaymentForPrepaidResearchFunding" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_MilestonePaymentForCertainIdentifiedTargets" name="MilestonePaymentForCertainIdentifiedTargets" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_MilestonePaymentForCertainOtherIdentifiedTargets" name="MilestonePaymentForCertainOtherIdentifiedTargets" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_PrepaidResearchFundingPaymentReceived" name="PrepaidResearchFundingPaymentReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_ClinicalAndRegulatoryMilestonePayables" name="ClinicalAndRegulatoryMilestonePayables" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_ClinicalAndRegulatoryMilestonesPayableUponInitiationOfPhaseTwoClinicalTrial" name="ClinicalAndRegulatoryMilestonesPayableUponInitiationOfPhaseTwoClinicalTrial" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="fulc_SalesMilestone" name="SalesMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="fulc_ClinicalAndRegulatoryMilestoneDueUponInitiationOfPhaseTwoClinicalTrialAchievedAndPaid" name="ClinicalAndRegulatoryMilestoneDueUponInitiationOfPhaseTwoClinicalTrialAchievedAndPaid" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="fulc_ClinicalAndRegulatoryMilestonesDueUponInitiationOfPhaseTwoClinicalTrialRecordedAsExpense" name="ClinicalAndRegulatoryMilestonesDueUponInitiationOfPhaseTwoClinicalTrialRecordedAsExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_GSKAgreementMember" name="GSKAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_AreaOfPropertiesHeldUnderOperatingLeases" name="AreaOfPropertiesHeldUnderOperatingLeases" type="num:areaItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="fulc_LesseeOperatingLeasesAllowanceForNormalLeaseholdImprovements" name="LesseeOperatingLeasesAllowanceForNormalLeaseholdImprovements" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_LesseeOperatingLeaseWrittenNoticePeriodToExtendAdditionalLeaseTerm" name="LesseeOperatingLeaseWrittenNoticePeriodToExtendAdditionalLeaseTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_CommitmentsAndContingenciesLineItems" name="CommitmentsAndContingenciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_CommitmentsAndContingenciesTable" name="CommitmentsAndContingenciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_CommitmentForOfficeAndLaboratorySpaceLeaseOfCurrentCorporateHeadquartersMember" name="CommitmentForOfficeAndLaboratorySpaceLeaseOfCurrentCorporateHeadquartersMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_LegalProceedingsMember" name="LegalProceedingsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_EffectiveIncomeTaxRateReconciliationFederalOrphanDrugCredits" name="EffectiveIncomeTaxRateReconciliationFederalOrphanDrugCredits" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_DeferredTaxAssetsOrphanDrugCreditCarryforwards" name="DeferredTaxAssetsOrphanDrugCreditCarryforwards" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_DeferredTaxAssetsLeaseIncentive" name="DeferredTaxAssetsLeaseIncentive" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_IncomeTaxesDisclosureLineItems" name="IncomeTaxesDisclosureLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_IncomeTaxesDisclosureTable" name="IncomeTaxesDisclosureTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_OperatingLossCarryforwardsExpirationYear" name="OperatingLossCarryforwardsExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_OperatingLossCarryforwardsIndefinitely" name="OperatingLossCarryforwardsIndefinitely" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="fulc_TaxCreditCarryforwardExpirationYear" name="TaxCreditCarryforwardExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_DefinedContributionPlanName" name="DefinedContributionPlanName" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="fulc_ThirdRockVenturesLLCMember" name="ThirdRockVenturesLLCMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="fulc_ConsultingServicesMember" name="ConsultingServicesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
