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Weighted average shares used to compute net loss per share, basic and diluted (in shares) Weighted Average Number of Shares Outstanding, Basic and Diluted Customer Customer [Axis] Beginning balance (in shares) Ending balance (in shares) Shares, Outstanding Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued or outstanding as of March 31, 2021 and December 31, 2020, respectively Preferred Stock, Value, Issued Income Taxes Income Tax, Policy [Policy Text Block] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Total operating expenses Operating Expenses Document Transition Report Document Transition Report Exercisable (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Merck Sharp & Dohme Corp Merck Sharp And Dohme Corporation [Member] Merck Sharp and Dohme Corporation. 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Collaboration Agreement Collaborative Arrangement [Member] Net proceeds from follow on offering Proceeds from Issuance of Common Stock Related Party Transaction [Axis] Related Party Transaction [Axis] Amendment Flag Amendment Flag Computation of Net Loss Per Common Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Total Operating Leases, Future Minimum Payments Due Net Loss per Share Earnings Per Share, Policy [Policy Text Block] 2021 Operating Leases, Future Minimum Payments, Remainder of Fiscal Year Vested and expected to vest (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested And Expected To Vest, Number of Shares Research and Development Research and Development Expense, Policy [Policy Text Block] Research Collaboration and License Agreements Collaborative Arrangement Disclosure [Text Block] Cash flows from investing activities Net Cash Provided by (Used in) Investing Activities [Abstract] Common stock, shares issued (in shares) Common Stock, Shares, Issued Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value 2022 Operating Leases, Future Minimum Payments, Year One Operating Leases, Future Minimum Payments, Year One Balance Sheet Components Supplemental Balance Sheet Disclosures [Text Block] Fair values of assets and services exchanged allocated to equity component Fair Value Of Assets And Services Exchanged Allocated To Equity Component Fair value of assets and services exchanged allocated to equity component. Net cash used in operating activities Net Cash Provided by (Used in) Operating Activities Entity Central Index Key Entity Central Index Key Measurement Frequency [Axis] Measurement Frequency [Axis] Personnel-related costs Accrued Personnel Expense Current Accrued personnel expense current. Merck Merck [Member] Merck. Deferred rent, current Deferred Rent Credit, Current Financial Instruments [Domain] Financial Instruments [Domain] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Total stockholders' equity Beginning balance Ending balance Stockholders' Equity Attributable to Parent Concentration Risk Type Concentration Risk Type [Domain] Upfront license fee Upfront License Fee Upfront license fee. Rent expense Operating Leases, Rent Expense, Net Total liabilities and stockholders' equity Liabilities and Equity Accumulated other comprehensive income (loss) Accumulated Other Comprehensive Income (Loss), Net of Tax Income Taxes Income Tax Disclosure [Text Block] Entity Address, City or Town Entity Address, City or Town Upon administration of an applicable product to the first patient in the first Phase 3 clinical trial for such product for the given indication Milestone Payment For Potential Achievement Of Certain Clinical Development Events Milestone payment for potential achievement of certain clinical development events. 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Entity Tax Identification Number Entity Tax Identification Number Revenue Recognition Revenue from Contract with Customer [Policy Text Block] Interest income, net Interest Income (Expense), Nonoperating, Net Collaborative arrangement funding, amount Collaborative Arrangement Annual Funding Amount Over Agreement Term Collaborative arrangement annual funding amount over agreement term. Net loss Net loss Net loss Net Income (Loss) Attributable to Parent Number of options to extend performance of research program Number Of Options To Extend Performance Of Research Program Number of options to extend performance of research program. Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Capitalized Contract Cost [Domain] Capitalized Contract Cost [Domain] Common stock remaining to be sold under open market sales agreement Sale of Stock, Common Stock Available For Issuance Sale of Stock, Common Stock Available For Issuance Other non-current assets Other Assets, Noncurrent Issuance of common stock under offering, net of issuance costs Stock Issued During Period, Value, New Issues Uses and Sources of Liquidity Uses And Sources Of Liquidity, Policy [Policy Text Block] Uses And Sources Of Liquidity, Policy Total assets Assets Scenario [Axis] Scenario [Axis] Total current assets Assets, Current Current Fiscal Year End Date Current Fiscal Year End Date Summary Of Significant Accounting Policies [Table] Summary Of Significant Accounting Policies [Table] Summary of significant accounting policies Operating expenses: Operating Expenses [Abstract] Title of 12(b) Security Title of 12(b) Security Accrued Liabilities Schedule of Accrued Liabilities [Table Text Block] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Weighted-average grant date fair value of stock options, granted (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value ESPP shares available for purchase Four Zero One K Matching Plan [Member] Four zero one k matching plan. 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Revenue Revenue Benchmark [Member] Additional Paid-In Capital Additional Paid-in Capital [Member] Money market funds Money Market Funds [Member] Antidilutive Securities, Name Antidilutive Securities, Name [Domain] Share-based Payment Arrangement Share-based Payment Arrangement [Member] Capitalized Contract Cost [Axis] Capitalized Contract Cost [Axis] Less: accumulated depreciation and amortization Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Class of Stock [Line Items] Class of Stock [Line Items] Japan JAPAN Related party contract asset Contract with Customer, Asset, after Allowance for Credit Loss, Current Clinical trials and research and development costs Accrued Clinical Trials And Research And Development Costs Current Accrued clinical trials and research and development costs current. 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