Generation Bio (GBIO) 10-K 24 Feb 22 2021 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Feb. 18, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Document Transition Report	false
Entity File Number	001-39319
Entity Registrant Name	GENERATION BIO CO.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-4301284
Entity Address, Address Line One	301 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	655-7500
Title of 12(b) Security	Common Stock, $0.0001 Par Value
Trading Symbol	GBIO
Security Exchange Name	NASDAQ
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	false
Entity Public Float			$ 1.2
Entity Common Stock, Shares Outstanding		57,001,152
Auditor Name	Ernst & Young LLP
Auditor Firm ID	42
Auditor Location	Boston, Massachusetts
Entity Central Index Key	0001733294
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 375,145	$ 62,889
Marketable securities		199,438
Prepaid expenses and other current assets	4,041	5,408
Total current assets	379,186	267,735
Property and equipment, net	25,886	23,781
Operating lease right-of-use assets	65,143
Restricted cash	5,692	2,051
Deferred offering costs	461	336
Other long-term assets	403	252
Total assets	476,771	294,155
Current liabilities:
Accounts payable	2,023	267
Accrued expenses and other current liabilities	12,177	10,953
Operating lease liability	4,608
Total current liabilities	18,808	11,220
Operating lease liability, net of current portion	76,217
Deferred rent, net of current portion		14,922
Total liabilities	95,025	26,142
Commitments and contingencies (Note 12)
Stockholders' equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized and no shares issued or outstanding at December 31, 2021 and December 31, 2020
Common stock, $0.0001 par value; 150,000,000 shares authorized at December 31, 2021 and December 31, 2020; 56,980,701 and 46,970,012 shares issued at December 31, 2021 and December 31, 2020, respectively; 56,969,618 and 46,291,877 shares outstanding at December 31, 2021 and December 31, 2020, respectively	6	5
Additional paid-in capital	689,866	456,974
Accumulated other comprehensive income		9
Accumulated deficit	(308,126)	(188,975)
Total stockholders' equity	381,746	268,013
Total liabilities and stockholders' equity	$ 476,771	$ 294,155

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value per share	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	56,980,701	46,970,012
Common stock, shares outstanding	56,969,618	46,291,877

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating expenses:
Research and development	$ 85,247	$ 58,532	$ 50,134
General and administrative	33,854	22,582	12,168
Total operating expenses	119,101	81,114	62,302
Loss from operations	(119,101)	(81,114)	(62,302)
Other (expense) income:
Other (expense) and interest income, net	(50)	591	985
Net loss and net loss attributable to common stockholders	$ (119,151)	$ (80,523)	$ (61,317)
Net loss per share attributable to common stockholders, basic	$ (2.12)	$ (2.95)	$ (12.96)
Net loss per share attributable to common stockholders, diluted	$ (2.12)	$ (2.95)	$ (12.96)
Weighted average common shares outstanding, basic	56,295,409	27,256,494	4,731,519
Weighted average common shares outstanding, diluted	56,295,409	27,256,494	4,731,519
Comprehensive loss:
Net loss	$ (119,151)	$ (80,523)	$ (61,317)
Other comprehensive loss:
Unrealized (losses) gains on marketable securities	(9)	9	9
Comprehensive loss	$ (119,160)	$ (80,514)	$ (61,308)

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Total
Beginning balance at Dec. 31, 2018	$ 115,593	$ 1	$ 5,552	$ (9)	$ (47,135)	$ (41,591)
Beginning balance, shares at Dec. 31, 2018	26,425,664	4,054,475
Issuance of common stock for license, shares		125,677
Issuance of common stock upon exercise of stock options			100			100
Issuance of common stock upon exercise of stock options, shares		34,740
Vesting of restricted common stock, shares		1,055,997
Stock-based compensation expense			4,207			4,207
Unrealized gains (losses) on marketable securities				9		9
Net loss					(61,317)	(61,317)
Ending balance at Dec. 31, 2019	$ 115,593	$ 1	9,859		(108,452)	(98,592)
Ending balance, shares at Dec. 31, 2019	26,425,664	5,270,889
Issuance of Series C convertible preferred stock, net of issuance costs of $2,640, shares	19,936,296
Issuance of Series C convertible preferred stock, net of issuance costs of $2,640	$ 108,832
Issuance of common stock upon exercise of stock options			3,567			3,567
Issuance of common stock upon exercise of stock options, shares		792,845
Vesting of restricted common stock, shares		1,028,795
Conversion of convertible preferred stock into common stock upon initial public offering		$ 3	224,422			224,425
Conversion of convertible preferred stock into common stock upon initial public offering, shares		27,094,085
Issuance of common stock upon initial public offering, net of issuance costs		$ 1	210,714			210,715
Issuance of common stock upon initial public offering, net of issuance costs, shares		12,105,263
Conversion of convertible preferred stock into common stock upon initial public offering, convertible preferred stock	$ (224,425)
Conversion of convertible preferred stock into common stock upon initial public offering, convertible preferred stock, shares	(46,361,960)
Stock-based compensation expense			8,412			8,412
Unrealized gains (losses) on marketable securities				9		9
Net loss					(80,523)	(80,523)
Ending balance at Dec. 31, 2020		$ 5	456,974	9	(188,975)	268,013
Ending balance, shares at Dec. 31, 2020		46,291,877
Issuance of common stock upon exercise of stock options			3,257			$ 3,257
Issuance of common stock upon exercise of stock options, shares		767,185				767,185
Vesting of restricted common stock, shares		672,049
Issuance of common stock upon initial public offering, net of issuance costs		$ 1	211,285			$ 211,286
Issuance of common stock upon initial public offering, net of issuance costs, shares		9,200,000
Issuance of common stock under other equity plans			493			493
Issuance of common stock under other equity plans, shares		38,507
Stock-based compensation expense			17,857			17,857
Unrealized gains (losses) on marketable securities				$ (9)		(9)
Net loss					(119,151)	(119,151)
Ending balance at Dec. 31, 2021		$ 6	$ 689,866		$ (308,126)	$ 381,746
Ending balance, shares at Dec. 31, 2021		56,969,618

Consolidated Statements of Co_2

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
IPO [Member]
Stock issuance of costs	$ 590	$ 3,185
Series C Convertible Preferred Stock [Member]
Stock issuance of costs		$ 2,640

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (119,151)	$ (80,523)	$ (61,317)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	17,857	8,412	4,207
Depreciation and amortization expense	4,532	3,432	1,864
Amortization (accretion) of premium (discount) on marketable securities, net	529	249	(317)
Loss (gain) on sale of property and equipment	237		(10)
Changes in operating assets and liabilities:
Tenant receivable		448	840
Prepaid expenses and other current assets	1,367	(2,831)	(1,863)
Other noncurrent assets	(781)	(252)
Accounts payable	1,756	(1,444)	761
Accrued expenses and other current liabilities	1,833	3,426	2,470
Deferred rent		(1,059)	13,019
Net cash used in operating activities	(91,821)	(70,142)	(40,346)
Cash flows from investing activities:
Purchases of property and equipment	(5,958)	(5,518)	(19,986)
Proceeds from sale of property and equipment	105		80
Purchases of marketable securities		(243,178)	(20,789)
Maturities of marketable securities	198,900	43,500	88,680
Net cash provided by (used in) investing activities	193,047	(205,196)	47,985
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs incurred and paid in current period		108,854
Proceeds from public offering of common stock, net of underwriting discounts and commissions	211,876	213,900
Payment of offering costs	(955)	(3,226)	(22)
Proceeds from exercise of stock options	3,750	3,567	100
Net cash provided by financing activities	214,671	323,095	78
Net increase in cash, cash equivalents and restricted cash	315,897	47,757	7,717
Cash, cash equivalents and restricted cash at beginning of period	64,940	17,183	9,466
Cash, cash equivalents and restricted cash at end of period	380,837	64,940	17,183
Supplemental disclosure of noncash investing and financing information:
Lease liabilities arising from obtaining right-of-use assets	34,806
Purchases of property and equipment included in accounts payable and accrued expenses	1,515	494	644
Offering costs included in accrued expenses	$ 55	295	$ 65
Conversion of convertible preferred stock to common stock		$ 224,425

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Generation Bio Co., or Generation Bio, was incorporated on October 21, 2016 as Torus Therapeutics, Inc. and subsequently changed its name to Generation Bio Co. Generation Bio Co. and its consolidated subsidiary, or the company, we, our or us, are innovating genetic medicines to provide durable, redosable treatments for potentially hundreds of millions of patients living with rare and prevalent diseases. Our non-viral genetic medicines platform incorporates our high-capacity DNA construct called closed-ended DNA, or ceDNA; our cell-targeted lipid nanoparticle delivery system, or ctLNP; and our highly scalable capsid-free manufacturing process that uses our proprietary cell-free rapid enzymatic synthesis, or RES, to produce ceDNA. Using our approach, we are developing novel genetic medicines to provide targeted delivery of genetic payloads that include large and multiple genes to a range of cell types across a broad array of diseases. We are also engineering our genetic medicines to be redosable, which may enable individualized patient titration to reach the desired therapeutic expression and to maintain efficacy throughout a patient’s life. We are headquartered in Cambridge, Massachusetts. We are subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, uncertainties regarding the timing and ability to complete the build-out of our manufacturing facility, the ability to establish clinical- and commercial-scale manufacturing processes and the ability to secure additional capital to fund operations. Programs currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization of a product. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if our development efforts are successful, it is uncertain when, if ever, we will realize significant revenue from product sales. In June 2020, we completed our initial public offering, or IPO, pursuant to which we issued and sold 12,105,263 shares of our common stock, including 1,578,947 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, resulting in net proceeds of $210.7 million, after deducting underwriting discounts and commissions and other offering expenses. Upon the closing of the IPO, all of our outstanding convertible preferred stock automatically converted into shares of common stock. In January 2021, we issued and sold 9,200,000 shares of our common stock, including 1,200,000 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, in a follow-on public offering, resulting in net proceeds of $211.3 million after deducting underwriting discounts and commissions and other offering expenses. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, we have funded our operations with proceeds from the sales of instruments convertible into convertible preferred stock (which converted into convertible preferred stock in 2017), the sale of convertible preferred stock (which converted into common stock in 2020), and most recently, with proceeds from the sale of common stock in underwritten public offerings. We have incurred recurring losses, including net losses of $119.2 million for the year ended December 31, 2021. As of December 31, 2021, we had an accumulated deficit of $308.1 million. We expect to continue to generate operating losses in the foreseeable future. As of February 24, 2022, the issuance date of these consolidated financial statements, we expect that our cash and cash equivalents will be sufficient to fund our operating expenses and capital expenditure requirements for at least 12 months. We will need to obtain additional funding through public or private equity offerings, debt financings, collaborations, strategic alliances and/or licensing arrangements. We may not be able to obtain financing on acceptable terms, or at all, and we may not be able to enter into collaborative or strategic alliances or licensing arrangements. The terms of any financing may adversely affect the holdings or the rights of our stockholders. Arrangements with collaborators or others may require us to relinquish rights to certain of our technologies or programs. If we are unable to obtain funding, we could be forced to delay, reduce or eliminate some or all of our research and development programs, pipeline expansion or commercialization efforts, which could adversely affect our business prospects. Although management will continue to pursue these plans, there is no assurance that we will be successful in obtaining sufficient funding on terms acceptable to us to fund continuing operations when needed or at all. The accompanying consolidated financial statements reflect the operations of Generation Bio and our wholly owned subsidiary, Generation Bio Securities Corporation. Intercompany balances and transactions have been eliminated in consolidation. The accompanying consolidated financial statements have been prepared in conformity with generally accepted accounting principles, or GAAP, in the United States of America. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification, or ASC, and Accounting Standards Update, or ASU, of the Financial Accounting Standards Board, or FASB.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and stock-based compensation expense. We base our estimates on historical experience, known trends and other market-specific or other relevant factors that we believe to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions. Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose us to concentrations of credit risk consist primarily of cash, cash equivalents, and marketable securities. We believe that we are not exposed to significant credit risk due to the financial strength of the depository institutions in which our cash and cash equivalents are held. We maintain our cash equivalents in money market funds that invest in U.S. treasury securities. We have adopted an investment policy that limits the amounts that we may invest in the securities of single issuer with the exclusion of the U.S. government. We have not experienced any credit losses. We did not have marketable securities as of December 31, 2021. We are dependent on a small number of third-party suppliers for our drug substance and drug product. In particular, we rely, and expect to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates we may develop for our programs. These programs could be adversely affected by a significant interruption in the supply process. Cash equivalents We consider all highly liquid investments with a remaining maturity when purchased of three months or less to be cash equivalents. Restricted cash Amounts included in restricted cash represent amounts pledged as collateral for letters of credit required for security deposits on our leased facilities. These amounts are classified as restricted cash on our consolidated balance sheets. Marketable securities Our marketable securities, which consisted of debt securities as of December 31, 2020, are classified as available-for-sale and are reported at fair value. Unrealized gains and losses on available-for-sale debt securities are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Effective January 1, 2021, we adopted ASU 2016-13, Financial Instruments – Credit Losses (Topic 326). As we have never recorded any other-than-temporary-impairment adjustments to our available-for-sale debt securities prior to January 1, 2021, no transition provisions are applicable to our consolidated financial statements and related disclosures. We assess our available-for-sale debt securities under the available-for-sale debt security impairment model in Topic 326 as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on our available-for-sale debt securities is the result of a credit loss. We record credit losses in the consolidated statements of operations and comprehensive loss as credit loss expense, which is limited to the difference between the fair value and the amortized cost of the security. To date, we have not recorded any credit losses on our available-for-sale debt securities. Fair value measurements Certain of our assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: ● Level 1—Quoted prices in active markets for identical assets or liabilities. ● Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. ● Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. Our cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of our accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. Property and equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: ​ ​ ​ ​ ​ Estimated Useful Life Laboratory equipment ​ 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life ​ Costs for capital assets not yet placed into service are capitalized as construction-in-progress and depreciated once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance which do not improve or extend the life of the respective assets are charged to expense as incurred. Impairment of long-lived assets Long-lived assets consist of property and equipment. We evaluate the recoverability of our long-lived assets when circumstances indicate that an event of impairment may have occurred. We recognize an impairment loss only if the carrying amount of a long-lived asset is not recoverable based on its undiscounted future cash flows. Impairment is measured based on the difference between the carrying value of the related assets and the fair value of such assets. We did not record any impairment losses on long-lived assets during the years ended December 31, 2021 or 2020. Leases We determine whether a contract is, or contains, a lease at inception. We classify each of our leases as operating or financing considering factors such as the length of the lease term, the present value of the lease payments, the nature of the asset being leased, and the potential for ownership of the asset to transfer during the lease term. Leases with terms greater than one-year are recognized on the consolidated balance sheets as right-of-use assets and lease liabilities and are measured at the present value of the fixed payments due over the expected lease term less the present value of any incentives, rebates or abatements we expect to receive from the lessor. Options to extend a lease are included in the expected lease term if exercise of the option is deemed reasonably certain. Costs determined to be variable and not based on an index or rate are not included in the measurement of the lease liability and are expensed as incurred. The interest rate implicit in lease contracts is typically not readily determinable. As such, we utilize the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis an amount equal to the lease payments over a similar term and in a similar economic environment. To estimate our incremental borrowing rate, a credit rating applicable to our company is estimated using a synthetic credit rating analysis since we do not currently have a rating agency-based credit rating. We record expense to recognize fixed lease payments on a straight-line basis over the expected lease term. We have elected the practical expedient not to separate lease and non-lease components for real estate leases. Classification and accretion of convertible preferred stock Our convertible preferred stock outstanding prior to the IPO was classified outside of stockholders’ equity (deficit) on the consolidated balance sheets because the holders of such shares had redemption rights in the event of a deemed liquidation that, in certain situations, was not solely within our control and would have required the redemption of the then-outstanding convertible preferred stock. Segment information We have determined that our chief executive officer is the chief operating decision maker, or CODM. The CODM reviews financial information presented on a consolidated basis. Resource allocation decisions are made by the CODM based on consolidated results. There are no segment managers who are held accountable by the CODM for operations, operating results, and planning for levels or components below the consolidated unit level. As such, we have concluded that we operate as one segment. All long-lived assets are located in the United States. Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock-based compensation, employee benefits, facilities costs, laboratory supplies, depreciation and amortization, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology with no alternative future use are expensed as research and development expenses in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Research and manufacturing contract costs and accruals We have entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and costs for research and development and manufacturing activities are expensed in the period in which they are incurred. Payments for such activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred. We record accruals for costs incurred but not billed. When evaluating the adequacy of the accrued liabilities, we analyze progress of the research studies and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from our estimates. Our historical accrual estimates have not been materially different from the actual costs. Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Stock-based compensation We measure stock options with service-based vesting or performance-based vesting granted to employees, non-employees and directors based on the fair value on the date of grant using the Black-Scholes option-pricing model. We measure restricted common stock awards and units using the difference between the purchase price per share of the award or unit, if any, and the fair value of our common stock at the date of grant. Compensation expense for awards with service-based vesting is generally recognized over the vesting period of the award using the straight-line method to record the expense. We use the graded-vesting method to record the expense of awards with both service-based and performance-based vesting conditions, commencing once achievement of the performance condition becomes probable. We account for forfeitures of share-based awards as they occur. We classify stock-based compensation expense in our consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Comprehensive loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2021 and 2020, our only element of other comprehensive loss was unrealized gains (losses) on available-for-sale debt securities. Income taxes We account for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in our tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. We assess the likelihood that our deferred tax assets will be recovered from future taxable income and, to the extent we believe, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. We account for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Our policy is to recognize interest and/or penalties related to income tax matters in income tax expense. We have accrued no amounts for interest and penalties on our consolidated balance sheets at December 31, 2021 and 2020. Net loss per share Prior to closing of the IPO, we followed the two-class method when computing net income (loss) per share as we had issued shares that met the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income (loss) available to common stockholders for the period to be allocated between common stock and participating securities based upon their respective rights to share in the earnings as if all income (loss) for the period had been distributed. Basic net income (loss) per share attributable to common stockholders was computed by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders was computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders was computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. Our participating securities contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in our losses. Accordingly, in periods in which we reported a net loss, such losses were not allocated to such participating securities. In periods in which we reported a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders was the same as basic net loss per share attributable to common stockholders, since dilutive common shares were not assumed to have been issued if their effect was anti-dilutive. Subsequent to the closing of our IPO, we only have one class of shares outstanding and basic net income (loss) per common share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per common share is computed by dividing net income (loss) by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock awards. For periods in which we report a net loss, diluted net loss per common share is the same as basic net loss per common share, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. We reported a net loss attributable to common stockholders for each of the years ended December 31, 2021, 2020 and 2019. Recently adopted accounting pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). The standard required that all lessees recognize the assets and liabilities that arise from leases on the balance sheet and disclose qualitative and quantitative information about its leasing arrangements. In July 2018, the FASB issued ASU 2018-11, which provided entities with an additional transition method to adopt Topic 842. Under the new transition method, an entity initially applied the new lease requirements at the adoption date, not the earliest period presented, and recognized a cumulative effect adjustment to the opening balance of retained earnings in the period of adoption. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. For nonpublic entities, the guidance was effective for annual reporting periods beginning after December 15, 2019. Early adoption was permitted for all entities. In November 2019, the FASB issued ASU 2019-10, which deferred the effective date for nonpublic entities to annual reporting periods beginning after December 15, 2020, and interim periods within fiscal years beginning after December 15, 2021. In June 2020, the FASB issued ASU 2020-05, which granted a one-year effective-date delay for nonpublic entities to annual reporting periods beginning after December 15, 2021 and to interim periods within fiscal years beginning after December 15, 2022. Early adoption continues to be allowed. We early adopted ASU 2016-02 on January 1, 2021 using the modified retrospective approach transition method as of the date of adoption such that prior periods are not restated. We elected a package of practical expedients, under which an entity need not reassess whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases, or initial direct costs for any existing leases. Please read Note 5 for additional disclosures related to accounting for leases under this standard. The adoption of the standard has had a material impact on our consolidated balance sheet as the standard requires us to measure and recognize a right-of-use asset and lease liability. As most leases do not provide an implicit rate, our incremental borrowing rate was determined based on the information available at the date of adoption to measure our lease liability. Costs determined to be variable and not based on an index or rate were not included in the measurement of the lease liability. We recognized a lease liability and related right-of-use asset on our consolidated balance sheet of approximately $49.7 million and $33.4 million, net of deferred rent, respectively, In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326). The standard adjusted the accounting for assets held at amortized costs basis, including marketable securities accounted for as available-for-sale. The standard eliminated the probable initial recognition threshold and required an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses was a valuation account that was deducted from the amortized cost basis of the financial assets to present the net amount expected to be collected. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2019 and for interim periods within those fiscal years. For nonpublic entities, the guidance was effective for annual reporting periods beginning after December 15, 2020. Early adoption was permitted for all entities. In November 2019, the FASB issued ASU 2019-10, which deferred the effective date for nonpublic entities to annual reporting periods beginning after December 15, 2022, including interim periods within those fiscal years. We adopted ASU 2016-13 on January 1, 2021, and the adoption did not have a material impact on our consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU No. 2019-12 , Income Taxes – Simplifying the Accounting for Income Taxes (Topic 740). The amendments in this update simplified the accounting for income taxes by removing certain exceptions to the general principles as well as clarifying and amending existing guidance to improve consistent application. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2020 and for interim periods within those fiscal years. For nonpublic entities, the guidance was effective for annual reporting periods beginning after December 15, 2021 and to interim periods within fiscal years beginning after December 15, 2022. Early adoption was permitted for all entities. Depending on the amendment, adoption may be applied on the retrospective, modified retrospective or prospective basis. Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that we adopt as of the specified effective date. Prior to December 31, 2021, we were an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012. As such, we elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which meant that when a standard was issued or revised and it had different application dates for public and nonpublic companies, we could have adopted the new or revised standard at the time nonpublic companies adopt the new or revised standard. We no longer qualify as an emerging growth company as of December 31, 2021, and thus, we will no longer be able to take advantage of the extended transition period for adopting new or revised accounting standards.

Marketable Securities And Fair

Marketable Securities And Fair Value Measurements	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Marketable securities and fair value measurements	3. Marketable securities and fair value measurements We did not have marketable securities as of December 31, 2021. ​ The following tables present our marketable securities by security type as of December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Gross ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair (in thousands) ​ Cost ​ Gains ​ Losses ​ Value U.S. treasury securities ​ $ 129,446 ​ $ 9 ​ $ (4) ​ $ 129,451 Commercial paper ​ 52,441 ​ — ​ — ​ 52,441 Corporate debt securities ​ 17,542 ​ 4 ​ — ​ 17,546 Total ​ $ 199,429 ​ $ 13 ​ $ (4) ​ $ 199,438 ​ Our marketable securities, as of December 31, 2020, consisted of investments that matured within one year. The following tables present our assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques that we utilized to determine such fair value: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2021 Using: (in thousands) ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 259,609 ​ $ — ​ $ — ​ $ 259,609 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 Using: (in thousands) Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ Money market funds ​ $ 63,827 ​ $ — ​ $ — ​ $ 63,827 Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. treasury securities ​ ​ — ​ ​ 129,451 ​ ​ — ​ ​ 129,451 Commercial paper ​ ​ — ​ ​ 52,441 ​ ​ — ​ ​ 52,441 Corporate debt securities ​ ​ — ​ ​ 17,546 ​ ​ — ​ ​ 17,546 Total ​ $ 63,827 ​ $ 199,438 ​ $ — ​ $ 263,265 ​ Money market funds were valued based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. Our marketable securities, which consisted of U.S. treasury securities, commercial paper and corporate debt securities were valued using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Property and equipment, net	4. Property and equipment, net Property and equipment, net consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, (in thousands) ​ 2021 ​ 2020 Laboratory equipment ​ $ 12,826 ​ $ 9,313 Computer equipment and software ​ 1,128 ​ 848 Furniture and fixtures ​ 826 ​ 826 Leasehold improvements ​ 17,374 ​ 17,374 Construction in progress ​ 3,748 ​ 1,082 ​ ​ ​ 35,902 ​ ​ 29,443 Less: Accumulated depreciation and amortization ​ ​ (10,016) ​ ​ (5,662) Total ​ $ 25,886 ​ $ 23,781 ​ Depreciation and amortization expense for the years ended December 31, 2021, 2020, and 2019 was $4.5 million, $3.4 million, and $1.9 million, respectively.

Leases

Leases	12 Months Ended
	Dec. 31, 2021
Leases [Abstract]
Leases	5. Leases We lease our office and laboratory space under a noncancelable operating lease that was entered into in August 2018, amended in July 2019 and June 2020, and expires in 2029, or the Office and Lab Lease. We have an option to extend the Office and Lab Lease term for one additional term of five years at the greater of the then-current base rent or the then-current fair market value. Exercise of this option was not determined to be reasonably certain and thus was not considered in determining the operating lease liability on the consolidated balance sheet as of December 31, 2021. We posted a letter of credit in the amount of approximately $2.1 million as a security deposit. The letter of credit is subject to increase if we were to sublease any portion of the leased premises. The Office and Lab Lease does not include any restrictions or covenants that had to be accounted for under the lease guidance. In July 2021, we entered into a 12-year noncancelable operating lease, or the Manufacturing Lease, to build out an approximately 104,000 square foot current Good Manufacturing Practice-, or cGMP-, compliant manufacturing facility in Waltham, Massachusetts intended for ceDNA manufacturing utilizing RES for cGMP-compliant clinical and initial commercial supply. In addition, the new facility is designed to provide expanded capacity for research production and process development activities. The Manufacturing Lease commenced in December 2021, when we were granted access to the facility and monthly rent payments are expected to begin in September 2022, and the total rent payment is expected to be approximately $104.3 million for the 12-year lease term. We have an option to extend the Manufacturing Lease term for two additional terms of five years each at the greater of the then-current base rent or the then-current fair market value. Exercise of this option was not determined to be reasonably certain and thus was not considered in determining the operating lease liability on the consolidated balance sheet as of December 31, 2021. In connection with the Manufacturing Lease, we provided a security deposit of $3.6 million in the form of a letter of credit. We will pay an initial monthly base rent of approximately $0.4 million that will increase annually, up to an estimated monthly base rent of $0.8 million. We are obligated to pay operating costs, taxes and utilities applicable to the facility. We will be responsible for costs of constructing interior improvements within the facility that exceed a construction allowance of $26.0 million provided by the landlord. The following table presents our costs included in operating expenses related to our noncancelable operating leases: ​ ​ ​ ​ ​ (in thousands) ​ ​ For the Year Ended December 31, 2021 Operating lease cost ​ $ 6,615 Variable lease cost ​ ​ 1,796 Total ​ $ 8,411 ​ Net cash paid for the amounts included in the measurement of the operating lease liability on the consolidated balance sheet and operating activities in our consolidated statement of cash flows was $7.1 million for the period ending December 31, 2021. The weighted-average remaining lease term and weighted-average incremental borrowing rate for all leases as of December 31, 2021 was 9.6 years and 7.0%, respectively. Future lease payments for our noncancelable operating leases as of December 31, 2021 and a reconciliation to the carrying amount of the operating lease liability presented in the consolidated balance sheet as of December 31, 2021 are as follows: ​ ​ ​ ​ Year Ending December 31, ​ (in thousands) 2022 ​ $ 9,198 2023 ​ 13,474 2024 ​ 15,252 2025 ​ 15,682 2026 ​ 16,134 Thereafter ​ 89,597 Total undiscounted payments due under operating leases ​ ​ 159,337 Total undiscounted unearned tenant improvements ​ ​ (26,000) Less imputed interest ​ ​ (52,512) Total ​ $ 80,825 ​ ​ ​ ​ Current operating lease liability ​ $ 4,608 Non-current operating lease liability ​ ​ 76,217 Total ​ $ 80,825 ​

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2021
Payables and Accruals [Abstract]
Accrued expenses	6. Accrued expenses Accrued expenses and other current liabilities consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2021 ​ December 31, 2020 (in thousands) ​ ​ ​ ​ ​ Accrued employee compensation and benefits ​ $ 7,579 ​ $ 6,150 Accrued external research and development expenses ​ 2,091 ​ 1,772 Accrued professional fees ​ 962 ​ 940 Property and equipment ​ ​ 869 ​ ​ 258 Deferred rent ​ — ​ 1,389 Other ​ 676 ​ 444 Total ​ $ 12,177 ​ $ 10,953 ​

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2021
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	7. Convertible preferred stock Prior to the IPO, we issued Series A convertible preferred stock, or Series A, Series B convertible preferred stock, or Series B, and Series C convertible preferred stock, or Series C. Collectively the Series A, Series B and Series C are referred to as the Preferred Stock. On January 9, 2020, we issued and sold 19,936,296 shares of Series C at a price of $5.5914 per share for gross proceeds of $111.5 million. We incurred issuance costs in connection with this transaction of $2.6 million. Upon issuance of each class of Preferred Stock, we assessed the embedded conversion and liquidation features of the shares and determined that such features did not require us to separately account for these features. We also concluded that no beneficial conversion feature existed on the issuance date of each class of Preferred Stock. Upon the closing of the IPO in June 2020, our Preferred Stock automatically converted into 27,094,085 shares of common stock.

Equity

Equity	12 Months Ended
	Dec. 31, 2021
Equity [Abstract]
Equity	8. Equity As of December 31, 2021, our amended and restated certificate of incorporation authorizes us to issue 150,000,000 shares of common stock, par value $0.0001 per share, and 5,000,000 shares of preferred stock, par value $0.0001 per share, all of which preferred stock is undesignated. In January 2021, we issued and sold 9,200,000 shares of our common stock, including 1,200,000 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, in a follow-on public offering, resulting in net proceeds of $211.3 million after deducting underwriting discounts and commissions and other offering expenses. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of our stockholders. Holders of common stock are not entitled to receive dividends, unless declared by the board of directors.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2021
Share-based Payment Arrangement [Abstract]
Stock-based compensation	9. Stock-based compensation Our 2017 Stock Incentive Plan, or the 2017 Plan, provided for us to grant incentive or nonstatutory stock options, restricted stock, restricted stock units and other equity awards to employees, non-employees and directors. In January 2020, the number of shares of common stock authorized for issuance under the 2017 Plan was increased from 8,407,405 shares to 10,275,717 shares. In May 2020 our board of directors adopted, and in June 2020 our stockholders approved, the 2020 Stock Incentive Plan, or the 2020 Plan, and, together with the 2017 Plan, or the Plans, which became effective on June 11, 2020. The 2020 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares of common stock reserved for issuance under the 2020 Plan is the sum of (1) 2,547,698 shares; plus (2) the number of shares (up to a maximum of 7,173,014 shares) as was equal to the sum of (x) the number of shares of common stock reserved for issuance under the 2017 Plan that remained available for grant under the 2017 Plan on June 11, 2020 and (y) the number of shares of common stock subject to outstanding awards granted under the 2017 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2021 and continuing until, and including, the fiscal year ending December 31, 2030, equal to the lesser of (i) 4% of the number of shares of common stock outstanding on such date, and (ii) an amount determined by the board of directors. In January 2021 and 2022, the number of shares of common stock authorized for issuance under the 2020 Plan was increased from 10,275,717 shares to 12,154,517 shares and from 12,154,517 shares to 14,433,745 shares, respectively. Upon the effectiveness of the 2020 Plan, we ceased granting additional awards under the 2017 Plan. The Plans are administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions on any award under the Plans are determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the Plans with service-based vesting conditions generally vest over four years and expire after ten years. The exercise price for stock options granted is not less than the fair value of common stock as of the date of grant. Prior to our IPO, fair value of common stock was determined by the board of directors. Subsequent to our IPO, fair value of common stock is based on quoted market prices. As of December 31, 2021, 2,650,622 shares remained available for future issuance under the 2020 Plan. Shares subject to outstanding awards granted under the Plans that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right will be available for future awards under the 2020 Plan. Stock option valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. We completed our IPO in 2020 and, accordingly, lack company-specific historical and implied volatility information. Therefore, we estimated our expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as we have adequate historical data regarding the volatility of our own traded stock price. For options with service-based vesting conditions, the expected term of our stock options was determined utilizing the “simplified” method for awards that qualified as “plain-vanilla” options. The risk-free interest rate was determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield was based on the fact that we have never paid cash dividends and do not expect to pay any cash dividends in the foreseeable future. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 ​ Risk-free interest rate 0.81 % 0.60 % 2.00 % Expected volatility 76.0 % 77.3 % 75.7 % Expected dividend yield — — ​ — ​ Expected term (in years) 6.0 6.0 ​ 6.0 ​ ​ The following table summarizes our stock option activity since December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Weighted ​ ​ ​ ​ ​ ​ Average ​ Average ​ Aggregate ​ ​ Number of ​ Exercise ​ Contractual ​ Intrinsic ​ Shares Price Term Value ​ ​ ​ ​ ​ ​ ​ (in years) ​ (in thousands) Outstanding as of December 31, 2020 5,241,770 ​ $ 9.89 8.69 ​ $ 99,467 Granted 2,151,439 ​ 28.70 ​ Exercised (767,185) ​ 4.25 ​ Forfeited (420,936) ​ 10.23 ​ Outstanding as of December 31, 2021 6,205,088 ​ $ 17.09 8.33 ​ $ 5,140 Vested and expected to vest as of December 31, 2021 5,654,048 ​ $ 18.25 8.34 ​ $ 4,081 Options exercisable as of December 31, 2021 1,762,448 ​ $ 10.07 7.49 ​ $ 2,575 ​ The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of our common stock for those stock options that had strike prices lower than the fair value of our common stock. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2021, 2020 and 2019 was $18.81, $10.41, and $4.46 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2021, 2020, and 2019 was $19.4 million, $20.0 million and $0.1 million, respectively. Restricted common stock units We did not issue restricted stock units in the year ended December 31, 2021. During the year ended December 31, 2020, we issued 26,242 restricted stock units with a fair value of $0.5 million. The total fair value of restricted common stock units vested during the years ended December 31, 2021 and 2020 was approximately $0.2 million and $0.3 million, respectively. Restricted common stock awards We did not issue restricted stock awards in the year ended December 31, 2021. The following table summarizes our restricted common stock awards activity since December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Average ​ Shares Grant Date Fair Value Unvested restricted common stock as of December 31, 2020 678,135 ​ $ 1.28 Issued — ​ — Vested (665,474) ​ 1.22 Forfeited (1,578) ​ ​ 4.59 Unvested restricted common stock as of December 31, 2021 11,083 ​ $ 4.59 ​ The total fair value of restricted common stock awards vested during the years ended December 31, 2021, 2020, and 2019 was approximately $17.7 million, $15.8 million, and $7.0 million, respectively. Employee stock purchase plan In May 2020 our board of directors adopted, and in June 2020 our stockholders approved, the 2020 Employee Stock Purchase Plan, or the 2020 ESPP, which became effective June 11, 2020. The 2020 ESPP is administered by our board of directors or by a committee appointed by the board of directors. The 2020 ESPP provides participating employees with the opportunity to purchase shares of common stock through payroll deductions made over the term of the offering. The per-share purchase price at the end of each offering period is equal to the lesser of 85% of the closing price of our common stock at the beginning or end of the offering period. The number of shares of common stock reserved for issuance under the 2020 ESPP automatically increases on the first day of each fiscal year, beginning with the fiscal year that commenced on January 1, 2021 and continuing for each fiscal year until, and including the fiscal year commencing on, January 1, 2030, in an amount equal to the lowest of (1) 1,302,157 shares of common stock, (2) 1% of the number of shares of common stock outstanding on such date, and (3) an amount determined by the board of directors. In January 2021 and 2022, the number of shares of common stock authorized for issuance under the 2020 ESPP was increased from 481,231 shares to 950,931 shares and from 950,931 shares to 1,520,738 shares, respectively. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of shares of common stock granted under the 2020 ESPP: ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2021 Risk-free interest rate 0.09 % Expected volatility 85.6 % Expected dividend yield — Expected term (in years) 0.5 ​ Stock-Based Compensation We record compensation cost for all share-based payment arrangements, including employee, non-employee and director stock options and restricted common stock. We recorded stock-based compensation expense in the following expense categories of our consolidated statements of operations and comprehensive loss: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ (in thousands) 2021 2020 ​ 2019 ​ Research and development expenses ​ $ 9,316 ​ $ 4,301 ​ $ 2,753 ​ General and administrative expenses ​ 8,541 ​ 4,111 ​ 1,454 ​ Total ​ $ 17,857 ​ $ 8,412 ​ $ 4,207 ​ ​ As of December 31, 2021, total unrecognized compensation cost related to unvested stock-based awards was $47.7 million, which is expected to be recognized over a weighted average period of 2.5 years. Additionally, as of December 31, 2021, we had unrecognized compensation cost related to unvested stock-based awards with performance-based vesting conditions for which performance has not been deemed probable of $1.8 million.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Income taxes	10. Income taxes For the years ended December 31, 2021, 2020 and 2019, we recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each year, due to the uncertainty of realizing a benefit from those items. All of our operating losses since inception have been generated in the United States. A reconciliation of the U.S. federal statutory income tax rate to our effective income tax rate is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2021 2020 2019 ​ Federal statutory income tax rate (21.0) % (21.0) % (21.0) % State income taxes, net of federal benefit (6.7) (7.0) ​ (5.9) ​ Federal and state research and development tax credits (2.4) (4.2) ​ (5.1) ​ Stock-based compensation expense (1.7) (2.6) ​ 1.0 ​ Other — — ​ 0.2 ​ Change in deferred tax asset valuation allowance 31.8 34.8 ​ 30.8 ​ Effective income tax rate — % — % — % ​ Net deferred tax assets as of December 31, 2021 and 2020 consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) 2021 2020 Deferred tax assets: ​ ​ Net operating loss carryforwards ​ $ 79,607 ​ $ 48,280 Research and development tax credit carryforwards ​ 12,157 ​ 9,385 Operating lease liability ​ ​ 22,082 ​ ​ — Deferred rent ​ — ​ 4,456 Other ​ 6,386 ​ 2,747 Total deferred tax assets ​ 120,232 ​ 64,868 Deferred tax liabilities: ​ ​ Operating lease right-of-use assets ​ ​ (17,797) ​ ​ — Property and equipment ​ (3,533) ​ (3,900) Total deferred tax liabilities ​ (21,330) ​ (3,900) Valuation allowance ​ ​ (98,902) ​ ​ (60,968) Net deferred tax assets ​ $ — ​ $ — ​ As of December 31, 2021, we had federal net operating loss carryforwards of $292.2 million, which may be available to offset future taxable income, of which $8.2 million of the total net operating loss carryforwards expire at various dates beginning in 2036, while the remaining $284.0 million do not expire but may be limited in their usage to an annual deduction equal to 80% of annual taxable income. As of December 31, 2021, we had state net operating loss carryforwards of $288.5 million, which may be available to offset future taxable income and expire at various dates beginning in 2036. In addition, as of December 31, 2021, we also had federal and state research and development tax credit carryforwards of $8.3 million and $4.9 million, respectively, which may be available to reduce future tax liabilities and expire at various dates beginning in 2036 and 2032, respectively. Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 and Section 383 of the Internal Revenue Code of 1986, as amended, or the Code, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income and tax liabilities. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three-year period. We have not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If we have experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or research and development tax credit carryforwards may be subject to an annual limitation, which is determined by first multiplying the value of our at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before their utilization. Further, until a study is completed by us and any limitation is known, no amounts are being presented as an uncertain tax position. Utilization of the research and development tax credit carryforwards may be subject to additional limitations. We have not yet conducted a study of our research and development credit carryforwards to determine if an adjustment to our carryforwards is required. Until a study is completed by us and any limitation is known, no amounts are being presented as an uncertain tax position. We have evaluated the positive and negative evidence bearing upon our ability to realize the deferred tax assets, which consist primarily of net operating loss carryforwards and research and development tax credit carryforwards. Management has considered our history of cumulative net losses incurred since inception, estimated future taxable income and prudent and feasible tax planning strategies and has concluded that it is more likely than not that we will not realize the benefits of federal and state net deferred tax assets. Accordingly, a full valuation allowance has been established against the net deferred tax assets as of December 31, 2021 and 2020. We reevaluate the positive and negative evidence at each reporting period. The changes in the valuation allowance for deferred tax assets during the year ended December 31, 2021 and 2020 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards. The changes in the valuation allowance for 2021 and 2020 were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, (in thousands) 2021 2020 Valuation allowance as of beginning of year ​ $ 60,968 ​ $ 33,113 Increases recorded to income tax provision ​ 37,934 ​ 27,855 Valuation allowance as of end of year ​ $ 98,902 ​ $ 60,968 ​ We assess the uncertainty in our income tax positions to determine whether a tax position of ours is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more-likely-than-not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than 50% likelihood of being realized upon the ultimate settlement with the relevant taxing authority. No reserve for uncertain tax positions or related interest and penalties have been recorded at December 31, 2021 and 2020. We file income tax returns as prescribed by the tax laws of the jurisdictions in which we operate. In the normal course of business, we are subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. We are open to future tax examination under statute from 2017 to the present. In March 2020, the Coronavirus Aid, Relief, and Economic Security Act, or the CARES Act, was signed into law making several changes to the Code. The changes include, but are not limited to, increasing the limitation on the amount of deductible interest expense, allowing companies to carryback certain net operating losses, and increasing the amount of net operating loss carryforwards that corporations can use to offset taxable income. The tax law changes in the CARES Act did not have a material impact on our income tax provision.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2021
License Agreements Disclosure [Abstract]
License Agreements	11. License agreements NIH We have an agreement with the U.S. Department of Health and Human Services, as represented by The National Heart, Lung, and Blood Institute, an Institute of the National Institutes of Health, or NIH, entered into in 2017, pursuant to which NIH granted us a non-exclusive license, with the right to grant sublicenses, under certain NIH intellectual property related to our ceDNA construct. In July 2019, the agreement was amended to include Association Institut de Myologie, Universite Pierre et Marie Curie, Centre National de la Recherche Scientifique, and Inserm Transfert SA, collectively referred to as the French Institutions, as a licensor. We are obligated to make future milestone payments of up to $0.4 million per licensed product upon the achievement of specified milestones as well as royalties on a licensed product-by-licensed product and country-by-country basis of a low single digit percentage of annual net sales of licensed products. We are obligated to pay a high single-digit royalty percentage of all sublicensing income. The royalties on net sales may be reduced by up to 25% in certain circumstances as defined in the agreement. Our royalty obligation expires on a licensed product-by-licensed product and country-by-country basis upon the expiration of the last-to-expire licensed intellectual property rights in such country. Additionally, we are required to reimburse the French Institutions for a portion of certain past and ongoing patent related expenses related to the licensed technology. The agreement requires us to use reasonable commercial efforts to meet certain performance milestones and execute a commercial development plan within specified timeframes. Unless terminated earlier, the agreement remains in effect until the last to expire of the licensed patent rights on a licensed product-by-licensed product and country-by-country basis. NIH and the French Institutions may terminate the agreement if we fail to perform our material obligations, including but not limited to our failure to meet the applicable performance milestones despite using commercially reasonable efforts, and have not remediated such deficiency within a specified time period. NIH and the French Institutions can terminate the agreement in the event we become insolvent, file a petition in bankruptcy, have such a petition filed against it, or determine to file a petition in bankruptcy. In addition, NIH and the French Institutions may terminate the agreement in the event of a material breach by us and failure to cure such breach within a certain period of time. We are currently in compliance with the terms of the agreement. We can voluntarily terminate the agreement with prior notice to NIH and the French Institutions. During each of the years ended December 31, 2021, 2020, and 2019, we recorded research and development expense of less than $0.1 million under this agreement. UMass We have an agreement with the University of Massachusetts as represented by and solely on behalf of its Medical School, or UMass, entered into in 2017, pursuant to which UMass granted us an exclusive license, with the right to grant sublicenses, under the UMass intellectual property related to our ceDNA construct. We are obligated to make future milestone payments of up to $0.8 million per licensed product upon the achievement of specified milestones as well as royalties on a licensed product-by-licensed product and country-by-country basis of a low single digit percentage of annual net sales of licensed products, subject to annual minimum royalties as defined in the agreement. Additionally, we have agreed to pay a low-to-mid single-digit royalty percentage of all sublicensing income, which will vary depending on when the sublicense agreement to a third party was executed. The milestone payments are non-refundable and non-creditable against any other payments due to UMass under the agreement. Our royalty obligation expires on a licensed product-by-licensed product and country-by-country basis upon the expiration of the last-to-expire licensed intellectual property rights in such country. The agreement requires us to use diligent efforts to meet certain performance milestones within specified timeframes. As part of the arrangement, we agreed to issue 125,677 shares of our common stock to UMass. We recorded $0.1 million as research and development expense and additional-paid-in capital at the time of the agreement in 2017, representing the fair value of the common stock at that time. The shares were issued in August 2019. Unless terminated earlier, the agreement will continue until the last-to-expire valid claim of the licensed patents. UMass may terminate the agreement if we fail to perform our material obligations, including but not limited to our failure to meet the applicable performance milestones despite using commercially reasonable efforts, and have not remediated such deficiency within a specified time period or negotiated a revised performance timeline. UMass can terminate the agreement if we fail to make any payments within a specified period after receiving written notice of such failure, or in the event of a material breach by us and failure to cure such breach within a certain period of time. We are currently in compliance with the terms of the agreement. We can voluntarily terminate the agreement with prior notice to UMass. As of December 31, 2021, we have paid the first milestone for the issuance of the first patent in the licensed patent application family, and we have recorded no royalty or other milestone liabilities under the UMass Agreement. During each of the years ended December 31, 2021, 2020, and 2019, we recorded research and development expense of less than $0.1 million under this agreement. Other license agreements We have other license agreements under which we may become subject to future additional fees and milestone payments.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and contingencies 401(k) Plan We have a defined-contribution plan under Section 401(k) of the Code, or the 401(k) Plan. The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. In September 2020, we adopted a match program, beginning on January 1, 2021, for employee contributions to the 401(k) Plan up to a maximum of four percent of the employee’s salary, subject to the maximums established under the Code. Indemnification agreements In the ordinary course of business, we may provide indemnification of varying scope and terms to vendors, lessors, contract research organizations, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, we have entered into indemnification agreements with members of our board of directors and our executive officers that will require us, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments we could be required to make under these indemnification agreements is, in many cases, unlimited. We have not incurred any material costs as a result of such indemnifications and are not currently aware of any indemnification claims. Legal proceedings We, from time to time, may be subject to legal proceedings and claims in the ordinary course of business. We are not currently aware of any such proceedings or claims that we believe will have, individually or in the aggregate, a material adverse effect on our business, financial condition or results of operations.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Net Loss per Share	13. Net loss per share Net loss per share We have generated a net loss in all periods presented, therefore the basic and diluted net loss per share attributable to common stockholders are the same as the inclusion of the potentially dilutive securities would be anti-dilutive. We excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ ​ 2021 2020 2019 ​ Convertible preferred stock (as converted to common stock) — — ​ 14,961,027 ​ Unvested restricted common stock 11,083 678,135 ​ 1,705,188 ​ Unvested restricted common stock units ​ 13,120 ​ 19,681 ​ — ​ Stock options to purchase common stock 6,205,088 5,241,770 ​ 3,373,884 ​ Total 6,229,291 5,939,586 ​ 20,040,099 ​ ​

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and stock-based compensation expense. We base our estimates on historical experience, known trends and other market-specific or other relevant factors that we believe to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions.
Concentrations of credit risk and of significant suppliers	Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose us to concentrations of credit risk consist primarily of cash, cash equivalents, and marketable securities. We believe that we are not exposed to significant credit risk due to the financial strength of the depository institutions in which our cash and cash equivalents are held. We maintain our cash equivalents in money market funds that invest in U.S. treasury securities. We have adopted an investment policy that limits the amounts that we may invest in the securities of single issuer with the exclusion of the U.S. government. We have not experienced any credit losses. We did not have marketable securities as of December 31, 2021. We are dependent on a small number of third-party suppliers for our drug substance and drug product. In particular, we rely, and expect to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates we may develop for our programs. These programs could be adversely affected by a significant interruption in the supply process.
Cash equivalents	Cash equivalents We consider all highly liquid investments with a remaining maturity when purchased of three months or less to be cash equivalents.
Restricted cash	Restricted cash Amounts included in restricted cash represent amounts pledged as collateral for letters of credit required for security deposits on our leased facilities. These amounts are classified as restricted cash on our consolidated balance sheets.
Marketable securities	Marketable securities Our marketable securities, which consisted of debt securities as of December 31, 2020, are classified as available-for-sale and are reported at fair value. Unrealized gains and losses on available-for-sale debt securities are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Effective January 1, 2021, we adopted ASU 2016-13, Financial Instruments – Credit Losses (Topic 326). As we have never recorded any other-than-temporary-impairment adjustments to our available-for-sale debt securities prior to January 1, 2021, no transition provisions are applicable to our consolidated financial statements and related disclosures. We assess our available-for-sale debt securities under the available-for-sale debt security impairment model in Topic 326 as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on our available-for-sale debt securities is the result of a credit loss. We record credit losses in the consolidated statements of operations and comprehensive loss as credit loss expense, which is limited to the difference between the fair value and the amortized cost of the security. To date, we have not recorded any credit losses on our available-for-sale debt securities.
Fair value measurements	Fair value measurements Certain of our assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: ● Level 1—Quoted prices in active markets for identical assets or liabilities. ● Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. ● Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. Our cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of our accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities.
Property and equipment	Property and equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: ​ ​ ​ ​ ​ Estimated Useful Life Laboratory equipment ​ 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life ​ Costs for capital assets not yet placed into service are capitalized as construction-in-progress and depreciated once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance which do not improve or extend the life of the respective assets are charged to expense as incurred.
Impairment of long-lived assets	Impairment of long-lived assets Long-lived assets consist of property and equipment. We evaluate the recoverability of our long-lived assets when circumstances indicate that an event of impairment may have occurred. We recognize an impairment loss only if the carrying amount of a long-lived asset is not recoverable based on its undiscounted future cash flows. Impairment is measured based on the difference between the carrying value of the related assets and the fair value of such assets. We did not record any impairment losses on long-lived assets during the years ended December 31, 2021 or 2020.
Leases	Leases We determine whether a contract is, or contains, a lease at inception. We classify each of our leases as operating or financing considering factors such as the length of the lease term, the present value of the lease payments, the nature of the asset being leased, and the potential for ownership of the asset to transfer during the lease term. Leases with terms greater than one-year are recognized on the consolidated balance sheets as right-of-use assets and lease liabilities and are measured at the present value of the fixed payments due over the expected lease term less the present value of any incentives, rebates or abatements we expect to receive from the lessor. Options to extend a lease are included in the expected lease term if exercise of the option is deemed reasonably certain. Costs determined to be variable and not based on an index or rate are not included in the measurement of the lease liability and are expensed as incurred. The interest rate implicit in lease contracts is typically not readily determinable. As such, we utilize the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis an amount equal to the lease payments over a similar term and in a similar economic environment. To estimate our incremental borrowing rate, a credit rating applicable to our company is estimated using a synthetic credit rating analysis since we do not currently have a rating agency-based credit rating. We record expense to recognize fixed lease payments on a straight-line basis over the expected lease term. We have elected the practical expedient not to separate lease and non-lease components for real estate leases.
Classification and accretion of convertible preferred stock	Classification and accretion of convertible preferred stock Our convertible preferred stock outstanding prior to the IPO was classified outside of stockholders’ equity (deficit) on the consolidated balance sheets because the holders of such shares had redemption rights in the event of a deemed liquidation that, in certain situations, was not solely within our control and would have required the redemption of the then-outstanding convertible preferred stock.
Segment information	Segment information We have determined that our chief executive officer is the chief operating decision maker, or CODM. The CODM reviews financial information presented on a consolidated basis. Resource allocation decisions are made by the CODM based on consolidated results. There are no segment managers who are held accountable by the CODM for operations, operating results, and planning for levels or components below the consolidated unit level. As such, we have concluded that we operate as one segment. All long-lived assets are located in the United States.
Research and development costs	Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock-based compensation, employee benefits, facilities costs, laboratory supplies, depreciation and amortization, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology with no alternative future use are expensed as research and development expenses in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Research and manufacturing contract costs and accruals	Research and manufacturing contract costs and accruals We have entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and costs for research and development and manufacturing activities are expensed in the period in which they are incurred. Payments for such activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred. We record accruals for costs incurred but not billed. When evaluating the adequacy of the accrued liabilities, we analyze progress of the research studies and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from our estimates. Our historical accrual estimates have not been materially different from the actual costs.
Patent costs	Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock-based compensation	Stock-based compensation We measure stock options with service-based vesting or performance-based vesting granted to employees, non-employees and directors based on the fair value on the date of grant using the Black-Scholes option-pricing model. We measure restricted common stock awards and units using the difference between the purchase price per share of the award or unit, if any, and the fair value of our common stock at the date of grant. Compensation expense for awards with service-based vesting is generally recognized over the vesting period of the award using the straight-line method to record the expense. We use the graded-vesting method to record the expense of awards with both service-based and performance-based vesting conditions, commencing once achievement of the performance condition becomes probable. We account for forfeitures of share-based awards as they occur. We classify stock-based compensation expense in our consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.
Comprehensive loss	Comprehensive loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2021 and 2020, our only element of other comprehensive loss was unrealized gains (losses) on available-for-sale debt securities.
Income taxes	Income taxes We account for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in our tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. We assess the likelihood that our deferred tax assets will be recovered from future taxable income and, to the extent we believe, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. We account for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Our policy is to recognize interest and/or penalties related to income tax matters in income tax expense. We have accrued no amounts for interest and penalties on our consolidated balance sheets at December 31, 2021 and 2020.
Net loss per share	Net loss per share Prior to closing of the IPO, we followed the two-class method when computing net income (loss) per share as we had issued shares that met the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income (loss) available to common stockholders for the period to be allocated between common stock and participating securities based upon their respective rights to share in the earnings as if all income (loss) for the period had been distributed. Basic net income (loss) per share attributable to common stockholders was computed by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders was computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders was computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. Our participating securities contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in our losses. Accordingly, in periods in which we reported a net loss, such losses were not allocated to such participating securities. In periods in which we reported a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders was the same as basic net loss per share attributable to common stockholders, since dilutive common shares were not assumed to have been issued if their effect was anti-dilutive. Subsequent to the closing of our IPO, we only have one class of shares outstanding and basic net income (loss) per common share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per common share is computed by dividing net income (loss) by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock awards. For periods in which we report a net loss, diluted net loss per common share is the same as basic net loss per common share, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. We reported a net loss attributable to common stockholders for each of the years ended December 31, 2021, 2020 and 2019.
Recently issued accounting pronouncements	Recently adopted accounting pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). The standard required that all lessees recognize the assets and liabilities that arise from leases on the balance sheet and disclose qualitative and quantitative information about its leasing arrangements. In July 2018, the FASB issued ASU 2018-11, which provided entities with an additional transition method to adopt Topic 842. Under the new transition method, an entity initially applied the new lease requirements at the adoption date, not the earliest period presented, and recognized a cumulative effect adjustment to the opening balance of retained earnings in the period of adoption. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. For nonpublic entities, the guidance was effective for annual reporting periods beginning after December 15, 2019. Early adoption was permitted for all entities. In November 2019, the FASB issued ASU 2019-10, which deferred the effective date for nonpublic entities to annual reporting periods beginning after December 15, 2020, and interim periods within fiscal years beginning after December 15, 2021. In June 2020, the FASB issued ASU 2020-05, which granted a one-year effective-date delay for nonpublic entities to annual reporting periods beginning after December 15, 2021 and to interim periods within fiscal years beginning after December 15, 2022. Early adoption continues to be allowed. We early adopted ASU 2016-02 on January 1, 2021 using the modified retrospective approach transition method as of the date of adoption such that prior periods are not restated. We elected a package of practical expedients, under which an entity need not reassess whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases, or initial direct costs for any existing leases. Please read Note 5 for additional disclosures related to accounting for leases under this standard. The adoption of the standard has had a material impact on our consolidated balance sheet as the standard requires us to measure and recognize a right-of-use asset and lease liability. As most leases do not provide an implicit rate, our incremental borrowing rate was determined based on the information available at the date of adoption to measure our lease liability. Costs determined to be variable and not based on an index or rate were not included in the measurement of the lease liability. We recognized a lease liability and related right-of-use asset on our consolidated balance sheet of approximately $49.7 million and $33.4 million, net of deferred rent, respectively, In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326). The standard adjusted the accounting for assets held at amortized costs basis, including marketable securities accounted for as available-for-sale. The standard eliminated the probable initial recognition threshold and required an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses was a valuation account that was deducted from the amortized cost basis of the financial assets to present the net amount expected to be collected. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2019 and for interim periods within those fiscal years. For nonpublic entities, the guidance was effective for annual reporting periods beginning after December 15, 2020. Early adoption was permitted for all entities. In November 2019, the FASB issued ASU 2019-10, which deferred the effective date for nonpublic entities to annual reporting periods beginning after December 15, 2022, including interim periods within those fiscal years. We adopted ASU 2016-13 on January 1, 2021, and the adoption did not have a material impact on our consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU No. 2019-12 , Income Taxes – Simplifying the Accounting for Income Taxes (Topic 740). The amendments in this update simplified the accounting for income taxes by removing certain exceptions to the general principles as well as clarifying and amending existing guidance to improve consistent application. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2020 and for interim periods within those fiscal years. For nonpublic entities, the guidance was effective for annual reporting periods beginning after December 15, 2021 and to interim periods within fiscal years beginning after December 15, 2022. Early adoption was permitted for all entities. Depending on the amendment, adoption may be applied on the retrospective, modified retrospective or prospective basis. Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that we adopt as of the specified effective date. Prior to December 31, 2021, we were an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012. As such, we elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which meant that when a standard was issued or revised and it had different application dates for public and nonpublic companies, we could have adopted the new or revised standard at the time nonpublic companies adopt the new or revised standard. We no longer qualify as an emerging growth company as of December 31, 2021, and thus, we will no longer be able to take advantage of the extended transition period for adopting new or revised accounting standards.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Summary of Property and equipment	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: ​ ​ ​ ​ ​ Estimated Useful Life Laboratory equipment ​ 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life

Marketable Securities And Fai_2

Marketable Securities And Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Summary of marketable securities	The following tables present our marketable securities by security type as of December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Gross ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair (in thousands) ​ Cost ​ Gains ​ Losses ​ Value U.S. treasury securities ​ $ 129,446 ​ $ 9 ​ $ (4) ​ $ 129,451 Commercial paper ​ 52,441 ​ — ​ — ​ 52,441 Corporate debt securities ​ 17,542 ​ 4 ​ — ​ 17,546 Total ​ $ 199,429 ​ $ 13 ​ $ (4) ​ $ 199,438
Summary of assets measured at fair value on a recurring basis	The following tables present our assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques that we utilized to determine such fair value: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2021 Using: (in thousands) ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 259,609 ​ $ — ​ $ — ​ $ 259,609 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 Using: (in thousands) Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ Money market funds ​ $ 63,827 ​ $ — ​ $ — ​ $ 63,827 Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. treasury securities ​ ​ — ​ ​ 129,451 ​ ​ — ​ ​ 129,451 Commercial paper ​ ​ — ​ ​ 52,441 ​ ​ — ​ ​ 52,441 Corporate debt securities ​ ​ — ​ ​ 17,546 ​ ​ — ​ ​ 17,546 Total ​ $ 63,827 ​ $ 199,438 ​ $ — ​ $ 263,265

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Summary of Property and equipment, net	Property and equipment, net consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, (in thousands) ​ 2021 ​ 2020 Laboratory equipment ​ $ 12,826 ​ $ 9,313 Computer equipment and software ​ 1,128 ​ 848 Furniture and fixtures ​ 826 ​ 826 Leasehold improvements ​ 17,374 ​ 17,374 Construction in progress ​ 3,748 ​ 1,082 ​ ​ ​ 35,902 ​ ​ 29,443 Less: Accumulated depreciation and amortization ​ ​ (10,016) ​ ​ (5,662) Total ​ $ 25,886 ​ $ 23,781

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2021
Leases [Abstract]
Costs included in operating expenses related to the operating lease	​ ​ ​ ​ ​ (in thousands) ​ ​ For the Year Ended December 31, 2021 Operating lease cost ​ $ 6,615 Variable lease cost ​ ​ 1,796 Total ​ $ 8,411
Future lease payments for noncancelable operating lease	Future lease payments for our noncancelable operating leases as of December 31, 2021 and a reconciliation to the carrying amount of the operating lease liability presented in the consolidated balance sheet as of December 31, 2021 are as follows: ​ ​ ​ ​ Year Ending December 31, ​ (in thousands) 2022 ​ $ 9,198 2023 ​ 13,474 2024 ​ 15,252 2025 ​ 15,682 2026 ​ 16,134 Thereafter ​ 89,597 Total undiscounted payments due under operating leases ​ ​ 159,337 Total undiscounted unearned tenant improvements ​ ​ (26,000) Less imputed interest ​ ​ (52,512) Total ​ $ 80,825 ​ ​ ​ ​ Current operating lease liability ​ $ 4,608 Non-current operating lease liability ​ ​ 76,217 Total ​ $ 80,825

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2021
Payables and Accruals [Abstract]
Schedule of Accrued Liabilities [Table Text Block]	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2021 ​ December 31, 2020 (in thousands) ​ ​ ​ ​ ​ Accrued employee compensation and benefits ​ $ 7,579 ​ $ 6,150 Accrued external research and development expenses ​ 2,091 ​ 1,772 Accrued professional fees ​ 962 ​ 940 Property and equipment ​ ​ 869 ​ ​ 258 Deferred rent ​ — ​ 1,389 Other ​ 676 ​ 444 Total ​ $ 12,177 ​ $ 10,953

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of Inputs Used in Black-Scholes Option Pricing Model	The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2021 2020 2019 ​ Risk-free interest rate 0.81 % 0.60 % 2.00 % Expected volatility 76.0 % 77.3 % 75.7 % Expected dividend yield — — ​ — ​ Expected term (in years) 6.0 6.0 ​ 6.0 ​
Summarizes the Company's stock option activity	The following table summarizes our stock option activity since December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Weighted ​ ​ ​ ​ ​ ​ Average ​ Average ​ Aggregate ​ ​ Number of ​ Exercise ​ Contractual ​ Intrinsic ​ Shares Price Term Value ​ ​ ​ ​ ​ ​ ​ (in years) ​ (in thousands) Outstanding as of December 31, 2020 5,241,770 ​ $ 9.89 8.69 ​ $ 99,467 Granted 2,151,439 ​ 28.70 ​ Exercised (767,185) ​ 4.25 ​ Forfeited (420,936) ​ 10.23 ​ Outstanding as of December 31, 2021 6,205,088 ​ $ 17.09 8.33 ​ $ 5,140 Vested and expected to vest as of December 31, 2021 5,654,048 ​ $ 18.25 8.34 ​ $ 4,081 Options exercisable as of December 31, 2021 1,762,448 ​ $ 10.07 7.49 ​ $ 2,575
summarizes the Company's restricted common stock activity	We did not issue restricted stock awards in the year ended December 31, 2021. The following table summarizes our restricted common stock awards activity since December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Average ​ Shares Grant Date Fair Value Unvested restricted common stock as of December 31, 2020 678,135 ​ $ 1.28 Issued — ​ — Vested (665,474) ​ 1.22 Forfeited (1,578) ​ ​ 4.59 Unvested restricted common stock as of December 31, 2021 11,083 ​ $ 4.59
Summary of stock-based compensation expense was classified in the statements of operations and comprehensive loss	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ (in thousands) 2021 2020 ​ 2019 ​ Research and development expenses ​ $ 9,316 ​ $ 4,301 ​ $ 2,753 ​ General and administrative expenses ​ 8,541 ​ 4,111 ​ 1,454 ​ Total ​ $ 17,857 ​ $ 8,412 ​ $ 4,207 ​
Employee Stock Purchase Plan 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of Inputs Used in Black-Scholes Option Pricing Model	​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2021 Risk-free interest rate 0.09 % Expected volatility 85.6 % Expected dividend yield — Expected term (in years) 0.5

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Summary of reconciliation of the U.S. federal statutory income tax rate	A reconciliation of the U.S. federal statutory income tax rate to our effective income tax rate is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2021 2020 2019 ​ Federal statutory income tax rate (21.0) % (21.0) % (21.0) % State income taxes, net of federal benefit (6.7) (7.0) ​ (5.9) ​ Federal and state research and development tax credits (2.4) (4.2) ​ (5.1) ​ Stock-based compensation expense (1.7) (2.6) ​ 1.0 ​ Other — — ​ 0.2 ​ Change in deferred tax asset valuation allowance 31.8 34.8 ​ 30.8 ​ Effective income tax rate — % — % — %
Summary of Net deferred tax assets	Net deferred tax assets as of December 31, 2021 and 2020 consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) 2021 2020 Deferred tax assets: ​ ​ Net operating loss carryforwards ​ $ 79,607 ​ $ 48,280 Research and development tax credit carryforwards ​ 12,157 ​ 9,385 Operating lease liability ​ ​ 22,082 ​ ​ — Deferred rent ​ — ​ 4,456 Other ​ 6,386 ​ 2,747 Total deferred tax assets ​ 120,232 ​ 64,868 Deferred tax liabilities: ​ ​ Operating lease right-of-use assets ​ ​ (17,797) ​ ​ — Property and equipment ​ (3,533) ​ (3,900) Total deferred tax liabilities ​ (21,330) ​ (3,900) Valuation allowance ​ ​ (98,902) ​ ​ (60,968) Net deferred tax assets ​ $ — ​ $ —
Summary of changes in the valuation allowance for deferred tax assets	​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, (in thousands) 2021 2020 Valuation allowance as of beginning of year ​ $ 60,968 ​ $ 33,113 Increases recorded to income tax provision ​ 37,934 ​ 27,855 Valuation allowance as of end of year ​ $ 98,902 ​ $ 60,968

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Summary of potential dilutive securities, presented based on amounts outstanding	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ ​ 2021 2020 2019 ​ Convertible preferred stock (as converted to common stock) — — ​ 14,961,027 ​ Unvested restricted common stock 11,083 678,135 ​ 1,705,188 ​ Unvested restricted common stock units ​ 13,120 ​ 19,681 ​ — ​ Stock options to purchase common stock 6,205,088 5,241,770 ​ 3,373,884 ​ Total 6,229,291 5,939,586 ​ 20,040,099 ​

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional information (Detail) - USD ($) $ in Thousands	Jan. 31, 2021	Jun. 16, 2020	Jan. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Date of incorporation				Oct. 21, 2016
Net proceeds from shares issued	$ 211,300		$ 211,300
Net loss				$ (119,151)	$ (80,523)	$ (61,317)
Accumulated deficit				$ (308,126)	$ (188,975)
Common Stock [Member]
Shares issued during the period	9,200,000		9,200,000	9,200,000	12,105,263
Common Stock [Member] | IPO [Member]
Shares issued during the period		12,105,263
Net proceeds from shares issued		$ 210,700
Common Stock [Member] | Over-Allotment Option [Member]
Shares issued during the period	1,200,000	1,578,947	1,200,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary Of Property And Equipment (Detail)	12 Months Ended
	Dec. 31, 2021
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Useful life	5 years
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Useful life	3 years
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Useful life	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Useful life	Shorter of remaining life of lease or useful life

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2021USD ($)segment	Dec. 31, 2020USD ($)	Jan. 01, 2021USD ($)
Accounting Policies [Abstract]
Number of operating segment | segment	1
Income Tax rate settlement	50.00%
Lease liability	$ 80,825		$ 49,700
Operating lease right-of-use assets	65,143		$ 33,400
Interest and penalties	$ 0	$ 0

Marketable Securities And Fai_3

Marketable Securities And Fair Value Measurements - Summary of marketable securities (Detail) - Fair Value, Recurring [Member] $ in Thousands	Dec. 31, 2020USD ($)
Marketable Securities [Line Items]
Amortized cost	$ 199,429
Gross unrealized gains	13
Gross unrealized losses	(4)
Estimated fair value	199,438
U.S. treasury securities
Marketable Securities [Line Items]
Amortized cost	129,446
Gross unrealized gains	9
Gross unrealized losses	(4)
Estimated fair value	129,451
Commercial paper
Marketable Securities [Line Items]
Amortized cost	52,441
Estimated fair value	52,441
Corporate debt securities
Marketable Securities [Line Items]
Amortized cost	17,542
Gross unrealized gains	4
Estimated fair value	$ 17,546

Marketable Securities And Fai_4

Marketable Securities And Fair Value Measurements - Summary of assets measured at fair value on a recurring basis (Detail) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Marketable securities:
Marketable securities		$ 199,438
Total		263,265
Level 1
Marketable securities:
Total		63,827
Level 2
Marketable securities:
Total		199,438
Money Market Funds
Cash equivalents:
Cash equivalents	$ 259,609	63,827
Money Market Funds | Level 1
Cash equivalents:
Cash equivalents	$ 259,609	63,827
U.S. treasury securities
Marketable securities:
Marketable securities		129,451
U.S. treasury securities | Level 2
Marketable securities:
Marketable securities		129,451
Commercial paper
Marketable securities:
Marketable securities		52,441
Commercial paper | Level 2
Marketable securities:
Marketable securities		52,441
Corporate debt securities
Marketable securities:
Marketable securities		17,546
Corporate debt securities | Level 2
Marketable securities:
Marketable securities		$ 17,546

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 35,902	$ 29,443
Less: Accumulated depreciation and amortization	(10,016)	(5,662)
Property and equipment, net	25,886	23,781
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	12,826	9,313
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,128	848
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	826	826
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	17,374	17,374
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,748	$ 1,082

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment, Net, by Type [Abstract]
Depreciation and amortization expense	$ 4,532	$ 3,432	$ 1,864

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2021USD ($)ft²	Dec. 31, 2021USD ($)	Oct. 31, 2021USD ($)
Lessee operating lease year of expiry		2029
Operating lease term	12 years	12 years
Total rent payments		$ 159,337	$ 104,300
Good Manufacturing Practice compliant manufacturing facility | ft²	104,000
Initial monthly base rent on operating lease	$ 400
Option to extend lease	two
Operating lease agreement option to renewal term	5 years
Construction allowance in operating lease	$ 26,000
Lease existence of option to extend		true
Operating lease agreement option to renewal term		5 years
Letter of credit for security deposit		$ 2,100
Operating lease payments		$ 7,100
Weighted-average incremental borrowing		7.00%
Weighted-average remaining lease term		9 years 7 months 6 days
Maximum
Estimated monthly base rent on operating lease	800
Letter of Credit
Security deposit	$ 3,600

Leases - Costs included in oper

Leases - Costs included in operating expenses related to the operating lease (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021USD ($)
Leases [Abstract]
Operating lease cost	$ 6,615
Variable lease cost	1,796
Total	$ 8,411

Leases - Future lease payments

Leases - Future lease payments for noncancelable operating lease (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Oct. 31, 2021	Jan. 01, 2021
Leases [Abstract]
2022	$ 9,198
2023	13,474
2024	15,252
2025	15,682
2026	16,134
Thereafter	89,597
Total undiscounted payments due under operating leases	159,337	$ 104,300
Total undiscounted unearned tenant improvements	(26,000)
Less imputed interest	(52,512)
Current operating lease liability	4,608
Non-current operating lease liability	76,217
Total	$ 80,825		$ 49,700

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 7,579	$ 6,150
Accrued external research and development expenses	2,091	1,772
Accrued professional fees	962	940
Property and equipment	869	258
Deferred rent		1,389
Other	676	444
Total	$ 12,177	$ 10,953

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 09, 2020	Dec. 31, 2020	Jun. 16, 2020
Common Stock [Member]
Temporary Equity [Line Items]
Shares issued upon conversion			27,094,085
Series C Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares issued during the period	19,936,296
Shares issued price	$ 5.5914
Gross proceeds	$ 111,500
Stock issuance of costs	$ 2,600	$ 2,640

Equity - Additional Information

Equity - Additional Information (Detail) $ / shares in Units, $ in Millions	Jan. 31, 2021USD ($)shares	Jun. 16, 2020shares	Aug. 31, 2021USD ($)	Jan. 31, 2021USD ($)shares	Dec. 31, 2021security$ / sharesshares	Dec. 31, 2020$ / sharesshares
Common stock, shares authorized					150,000,000	150,000,000
Common stock, par value per share | $ / shares					$ 0.0001	$ 0.0001
Preferred stock, shares authorized					5,000,000	5,000,000
Preferred stock, par value per share | $ / shares					$ 0.0001	$ 0.0001
Net proceeds from shares issued | $	$ 211.3			$ 211.3
Number of vote per common share | security					1
Maximum
Aggregate gross offering price | $			$ 250
Common Stock [Member]
Shares issued during the period	9,200,000			9,200,000	9,200,000	12,105,263
Over-Allotment Option [Member] | Common Stock [Member]
Shares issued during the period	1,200,000	1,578,947		1,200,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jun. 11, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Jan. 31, 2022	Jan. 31, 2021	Jan. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of options granted		2,151,439
Per-share purchase price, percentage		85.00%
Unrecognized stock compensation on account of unvested options		$ 47.7
Unrecognized stock compensation on account of unvested options remaining period over which the compensation is to be recognised		2 years 6 months
Weighted average grant date fair value of Unvested restricted common stock, Vested | $ / shares		$ 1.22
No of Intrinsic stock option exercised		$ 19.4	$ 20	$ 0.1
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value		$ 0.2	$ 0.3
Equity Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of Unvested restricted common stock, Vested | $ / shares		$ 18.81	$ 10.41	$ 4.46
Unvested Stock Awards for Which Performance is not Probable
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock compensation on account of unvested options		$ 1.8
Service Based Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of options granted			26,242
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value			$ 0.5
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted common stock		$ 17.7	$ 15.8	$ 7
2020 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance	2,547,698
Stock based compensation vesting period service based		4 years
Stock based compensation period of expiry		10 years
Common stock shares available for future issuance		2,650,622
2020 Stock Incentive Plan | Maximum | Additional From 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance	7,173,014
2020 Stock Incentive Plan | Maximum | Annual Increase 2021 To 2030
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of common stock shares additionally reserved for issuance	4.00%
Employee Stock Purchase Plan 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance		950,931	481,231		1,520,738	950,931
Employee Stock Purchase Plan 2020 | Maximum | Annual Increase 2021 To 2030
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of common stock shares additionally reserved for issuance	1.00%
Number of share instruments newly issued under a share-based compensation plan	1,302,157
Common Stock [Member] | 2017 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of shares authorized for issuance				8,407,405			10,275,717
Common Stock [Member] | 2020 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of shares authorized for issuance		12,154,517	10,275,717		14,433,745	12,154,517

Stock-based compensation - Summ

Stock-based compensation - Summary of Inputs Used in Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.81%	0.60%	2.00%
Expected volatility	76.00%	77.30%	75.70%
Expected term (in years)	6 years	6 years	6 years
Employee Stock Purchase Plan 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.09%
Expected volatility	85.60%
Expected term (in years)	6 months

Stock-based compensation - Su_2

Stock-based compensation - Summarizes the Company's Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Number of shares Outstanding , Beginning	5,241,770
Number of Shares, Issued, Shares	2,151,439
Number of shares | Exercised	(767,185)
Number of shares | Forfeited	(420,936)
Number of shares Outstanding , Ending	6,205,088	5,241,770
Number of shares | Vested and expected to vest	5,654,048
Number of shares | Options exercisable	1,762,448
Weighted average exercise price Outstanding , Beginning	$ 9.89
Weighted average exercise price | Granted	28.70
Weighted average exercise price | Exercised	4.25
Weighted average exercise price | Forfeited	10.23
Weighted average exercise price Outstanding , Ending	17.09	$ 9.89
Weighted average exercise price | Vested and expected to vest	18.25
Weighted average exercise price | Options exercisable	$ 10.07
Weighted average contractual term (in years) Outstanding	8 years 3 months 29 days	8 years 8 months 8 days
Weighted average contractual term (in years) | Vested and expected to vest	8 years 4 months 2 days
Weighted average contractual term (in years) | Options exercisable	7 years 5 months 26 days
Aggregate intrinsic value (in thousands) Outstanding	$ 5,140	$ 99,467
Aggregate intrinsic value (in thousands) | Vested and expected to vest	4,081
Aggregate intrinsic value (in thousands) | Options exercisable	$ 2,575

Stock-based compensation - Su_3

Stock-based compensation - Summarizes the Company's Restricted Common Stock Activity (Detail)	12 Months Ended
	Dec. 31, 2021$ / sharesshares
Share-based Payment Arrangement [Abstract]
Unvested restricted common stock, Beginning | Shares	678,135
Number of Shares, Issued, Shares	2,151,439
Vested | Shares	(665,474)
Forfeited | Shares	(1,578)
Unvested restricted common stock, Ending | Shares	11,083
Weighted average grant date fair value of Unvested restricted common stock, Beginning | $ / shares | $ / shares	$ 1.28
Weighted average grant date fair value of Unvested restricted common stock, Vested | $ / shares | $ / shares	1.22
Weighted average grant date fair value of Unvested restricted common stock, Forfeited | $ / shares | $ / shares	4.59
Weighted average grant date fair value of Unvested restricted common stock, Ending | $ / shares | $ / shares	$ 4.59

Stock-Based Compensation - Su_4

Stock-Based Compensation - Summary of Stock-Based Compensation Expense was Classified in the Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 17,857	$ 8,412	$ 4,207
Research and development expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	9,316	4,301	2,753
General and administrative expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 8,541	$ 4,111	$ 1,454

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
IncomeTax [Line Items]
Income tax benefits	$ 0	$ 0	$ 0
Net operating loss carry forwards	$ 284,000
Percentage on annual taxable income	80.00%
Tax Credit Carryforward, Amount	$ 4,900
Percentage of change in ownership	50.00%
Unrecognized Tax Benefits of income tax rate	50.00%
Tax Credit Carry forward, Expiration Year	2036
Operating Loss Carry forwards, Expiration Year	2036
Loss from operations	$ (119,101)	(81,114)	$ (62,302)
Reserve for uncertain tax positions	0
Interest and penalties	0	$ 0
Federal income tax Authority [Member]
IncomeTax [Line Items]
Net operating loss carry forwards	292,200
State and Local Jurisdiction [Member]
IncomeTax [Line Items]
Net operating loss carry forwards	288,500
Federal and state research and development tax credit carry forwards [Member]
IncomeTax [Line Items]
Operating loss carryforward with expiration date	8,200
Tax Credit Carryforward, Amount	$ 8,300

Income Taxes - Summary of Recon

Income Taxes - Summary of Reconciliation of the U.S. Federal Statutory Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	(21.00%)	(21.00%)	(21.00%)
State income taxes, net of federal benefit	(6.70%)	(7.00%)	(5.90%)
Federal and state research and development tax credits	(2.40%)	(4.20%)	(5.10%)
Stock-based compensation expense	(1.70%)	(2.60%)	1.00%
Other		0.00%	0.20%
Change in deferred tax asset valuation allowance	31.80%	34.80%	30.80%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 79,607	$ 48,280
Research and development tax credit carryforwards	12,157	9,385
Operating lease liability	22,082
Deferred rent		4,456
Other	6,386	2,747
Total deferred tax assets	120,232	64,868
Deferred tax liabilities:
Operating lease right-of-use assets	(17,797)
Property and equipment	(3,533)	(3,900)
Total deferred tax liabilities	(21,330)	(3,900)
Valuation allowance	(98,902)	(60,968)	$ (33,113)
Total deferred tax assets	$ 0	$ 0

Income Taxes - Summary of Chang

Income Taxes - Summary of Changes in the Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Valuation Allowance [Abstract]
Valuation allowance as of beginning of year	$ 60,968	$ 33,113
Increases recorded to income tax provision	37,934	27,855
Valuation allowance as of end of year	$ 98,902	$ 60,968

License Agreements - Additional

License Agreements - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
License Agreements Disclosure [Line Items]
Research and development	$ 85,247	$ 58,532	$ 50,134
NIH [Member]
License Agreements Disclosure [Line Items]
Research and development	100	100	100
NIH [Member] | License Agreement Terms [Member]
License Agreements Disclosure [Line Items]
Future milestone obligation	$ 400
Percentage of increase decrease in net sales	25.00%
UMass [Member] | License Agreement Terms [Member]
License Agreements Disclosure [Line Items]
Future milestone obligation	$ 800
Shares Issued For Services	125,677
Research and development	$ 100	$ 100	$ 100
Adjustments to Additional Paid in Capital, Other	$ 100

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Defined contribution plan, percentage	4.00%

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potential Dilutive Securities, Presented Based on Amounts Outstanding (Detail) - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	6,229,291	5,939,586	20,040,099
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share			14,961,027
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	11,083	678,135	1,705,188
Unvested restricted common stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	13,120	19,681
Stock option to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	6,205,088	5,241,770	3,373,884