Generation Bio (GBIO) 10-Q 10 Nov 21 2021 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2021	Oct. 29, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity Interactive Data Current	Yes
Entity Central Index Key	0001733294
Current Fiscal Year End Date	--12-31
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2021
Entity Registrant Name	GENERATION BIO CO.
Trading Symbol	GBIO
Entity File Number	001-39319
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-4301284
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
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
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, $0.0001 Par Value
Entity Common Stock, Shares Outstanding		56,909,764
Entity Ex Transition Period	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 388,347	$ 62,889
Marketable securities	10,016	199,438
Prepaid expenses and other current assets	4,907	5,408
Total current assets	403,270	267,735
Property and equipment, net	22,962	23,781
Operating lease right-of-use assets	31,252
Restricted cash	5,692	2,051
Deferred offering costs	472	336
Other long-term assets	385	252
Total assets	464,033	294,155
Current liabilities:
Accounts payable	520	267
Accrued expenses and other current liabilities	9,627	10,953
Operating lease liability	4,359
Total current liabilities	14,506	11,220
Operating lease liability, net of current portion	42,317
Deferred rent, net of current portion		14,922
Total liabilities	56,823	26,142
Commitments and contingencies (Note 9)
Stockholders' equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized and no shares issued or outstanding at September 30, 2021 and December 31, 2020
Common stock, $0.0001 par value; 150,000,000 shares authorized at September 30, 2021 and December 31, 2020; 56,878,032 and 46,970,012 shares issued at September 30, 2021 and December 31, 2020, respectively; 56,732,465 and 46,291,877 shares outstanding at September 30, 2021 and December 31, 2020, respectively	6	5
Additional paid-in capital	684,387	456,974
Accumulated other comprehensive income		9
Accumulated deficit	(277,183)	(188,975)
Total stockholders' equity	407,210	268,013
Total liabilities and stockholders' equity	$ 464,033	$ 294,155

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 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	56,878,032	46,970,012
Common stock, shares outstanding	56,732,465	46,291,877

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating expenses:
Research and development	$ 21,991	$ 15,308	$ 63,400	$ 42,158
General and administrative	9,667	5,661	24,755	14,611
Total operating expenses	31,658	20,969	88,155	56,769
Loss from operations	(31,658)	(20,969)	(88,155)	(56,769)
Other (loss) income:
Other (expense) income, net	(197)	120	(53)	472
Net loss and net loss attributable to common stockholders	$ (31,855)	$ (20,849)	$ (88,208)	$ (56,297)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.56)	$ (0.46)	$ (1.57)	$ (2.68)
Weighted average common shares outstanding, basic and diluted	56,629,193	45,468,838	56,108,492	21,011,439
Comprehensive loss:
Net loss	$ (31,855)	$ (20,849)	$ (88,208)	$ (56,297)
Other comprehensive loss:
Unrealized losses on marketable securities	(3)	30	(8)	26
Comprehensive loss	$ (31,858)	$ (20,819)	$ (88,216)	$ (56,271)

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)	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, 2019	$ 115,593,000	$ 1,000	$ 9,859,000		$ (108,452,000)	$ (98,592,000)
Beginning balance, shares at Dec. 31, 2019	26,425,664	5,270,889
Issuance of common stock upon exercise of stock options			1,229,000			1,229,000
Issuance of common stock upon exercise of stock options, shares		319,642
Vesting of restricted common stock, shares		769,878
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	$ 108,832,000
Conversion of convertible preferred stock into common stock upon initial public offering, convertible preferred stock	$ (224,425,000)
Conversion of convertible preferred stock into common stock upon initial public offering, convertible preferred stock, shares	(46,361,960)
Conversion of convertible preferred stock into common stock upon initial public offering		$ 3,000	224,422,000			224,425,000
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		$ 1,000	210,714,000			210,715,000
Issuance of common stock upon initial public offering, net of issuance costs, shares		12,105,263
Stock-based compensation expense			5,033,000			5,033,000
Unrealized gains (losses) on marketable securities				$ 26,000		26,000
Net loss					(56,297,000)	(56,297,000)
Ending balance at Sep. 30, 2020		$ 5,000	451,257,000	26,000	(164,749,000)	286,539,000
Ending balance, shares at Sep. 30, 2020		45,559,757
Beginning balance at Jun. 30, 2020		$ 5,000	448,052,000	(4,000)	(143,900,000)	304,153,000
Beginning balance, shares at Jun. 30, 2020		45,028,024
Issuance of common stock upon exercise of stock options		$ 279,379	1,044,000			1,044,000
Vesting of restricted common stock, shares		252,354
Stock-based compensation expense			2,161,000			2,161,000
Unrealized gains (losses) on marketable securities				30,000		30,000
Net loss					(20,849,000)	(20,849,000)
Ending balance at Sep. 30, 2020		$ 5,000	451,257,000	26,000	(164,749,000)	286,539,000
Ending balance, shares at Sep. 30, 2020		45,559,757
Beginning balance at Dec. 31, 2020		$ 5,000	456,974,000	9,000	(188,975,000)	268,013,000
Beginning balance, shares at Dec. 31, 2020		46,291,877
Issuance of common stock upon exercise of stock options			2,947,000			2,947,000
Issuance of common stock upon exercise of stock options, shares		689,166
Vesting of restricted common stock, shares		535,911
Issuance of common stock upon initial public offering, net of issuance costs		$ 1,000	211,285,000			211,286,000
Issuance of common stock upon initial public offering, net of issuance costs, shares		9,200,000
Issuance of common stock under other equity plans			355,000			355,000
Issuance of common stock under other equity plans, shares		15,511
Stock-based compensation expense			12,826,000			12,826,000
Unrealized gains (losses) on marketable securities				(9,000)		(9,000)
Net loss					(88,208,000)	(88,208,000)
Ending balance at Sep. 30, 2021		$ 6,000	684,387,000		(277,183,000)	407,210,000
Ending balance, shares at Sep. 30, 2021		56,732,465
Beginning balance at Jun. 30, 2021		$ 6,000	678,720,000	4,000	(245,328,000)	433,402,000
Beginning balance, shares at Jun. 30, 2021		56,431,513
Issuance of common stock upon exercise of stock options			697,000			697,000
Issuance of common stock upon exercise of stock options, shares		145,646
Vesting of restricted common stock, shares		155,306
Stock-based compensation expense			4,970,000			4,970,000
Unrealized gains (losses) on marketable securities				$ (4,000)		(4,000)
Net loss					(31,855,000)	(31,855,000)
Ending balance at Sep. 30, 2021		$ 6,000	$ 684,387,000		$ (277,183,000)	$ 407,210,000
Ending balance, shares at Sep. 30, 2021		56,732,465

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
IPO [Member]
Stock issuance of costs	$ 590	$ 3,185
Series C Convertible Preferred Stock [Member]
Stock issuance of costs		$ 2,640

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (88,208)	$ (56,297)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	12,826	5,033
Depreciation and amortization expense	3,367	2,441
Amortization (accretion) of premium (discount) on marketable securities, net	514	88
Loss on sale of property and equipment	223
Changes in operating assets and liabilities:
Tenant receivable		448
Prepaid expenses and other current assets	504	(2,870)
Other noncurrent assets	(1,021)	(169)
Accounts payable	253	(1,321)
Accrued expenses and other current liabilities	523	1,631
Deferred rent		(761)
Net cash used in operating activities	(71,019)	(51,777)
Cash flows from investing activities:
Purchases of property and equipment	(3,123)	(3,724)
Proceeds from sale of property and equipment	105
Purchases of marketable securities		(221,823)
Maturities of marketable securities	188,900
Net cash provided by (used in) investing activities	185,882	(225,547)
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	(942)	(3,185)
Proceeds from exercise of stock options	3,302	1,229
Net cash provided by financing activities	214,236	320,798
Net increase in cash, cash equivalents and restricted cash	329,099	43,474
Cash, cash equivalents and restricted cash at beginning of period	64,940	17,183
Cash, cash equivalents and restricted cash at end of period	394,039	60,657
Supplemental disclosure of noncash investing and financing information:
Conversion of convertible preferred stock to common stock		224,425
Purchases of property and equipment included in accounts payable and accrued expenses	249	$ 404
Offering costs included in accrued expenses	$ 80

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 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 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 medicine 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 tissues 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 condensed 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 $88.2 million for the nine months ended September 30, 2021 and $56.3 million for the nine months ended September 30, 2020. As of September 30, 2021, we had an accumulated deficit of $277.2 million. We expect to continue to generate operating losses in the foreseeable future. As of November 10, 2021, the issuance date of these condensed consolidated financial statements, we expect that our cash, cash equivalents and marketable securities 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 condensed 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 condensed consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States of America, or GAAP. 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	9 Months Ended
	Sep. 30, 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 condensed 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. Unaudited interim financial information The condensed consolidated balance sheet as of December 31, 2020 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited financial statements as of September 30, 2021 and for the three and nine months ended September 30, 2021 and 2020 have been prepared by us pursuant to the rules and regulations of the Securities and Exchange Commission, or SEC, for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These financial statements should be read in conjunction with our audited financial statements included in our Annual Report on Form 10-K that was most recently filed with the SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair presentation of our financial position as of September 30, 2021, the results of operations for the three and nine months ended September 30, 2021 and 2020, and cash flows for the nine months ended September 30, 2021 and 2020 have been made. The results of operations for the three and nine months ended September 30, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021 or any other period. 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. Our marketable securities as of September 30, 2021 consisted of U.S. government 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 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 in the event of a significant interruption in the supply process. Marketable securities Our marketable securities that consist of debt securities as of September 30, 2021 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. Effective January 1, 2021, we adopted ASU 2016-13, Financial Instruments – Credit Losses (Topic 326), using the effective date method. 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 condensed 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 condensed 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. 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 condensed 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. Recently adopted accounting pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). The standard requires 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 applies the new lease requirements at the adoption date, not the earliest period presented, and recognizes 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 will not be 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 new standard. The adoption of the new standard has had a material impact on our condensed 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 condensed consolidated balance sheet of approximately $49.7 million and $33.4 million, net of deferred rent, respectively, as of January 1, 2021, which are presented as separate line items on the condensed consolidated balance sheet as of September 30, 2021. The adoption of the standard did not have a material impact on our condensed consolidated statement of operations and comprehensive loss and did not require a cumulative adjustment to accumulated deficit on our condensed consolidated statement of stockholders’ equity as of September 30, 2021. In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326). The new standard adjusts the accounting for assets held at amortized costs basis, including marketable securities accounted for as available-for-sale. The standard eliminates the probable initial recognition threshold and requires an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses is a valuation account that is 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 is 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 condensed 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 simplify 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 is effective for annual reporting periods beginning after December 15, 2021 and to interim periods within fiscal years beginning after December 15, 2022. Early adoption is 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. As an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012, we elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, we could adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard. However, as of June 30, 2021, the market value of our common stock that was held by non-affiliates exceeded $700.0 million, and as a result, we will no longer qualify as an emerging growth company as of December 31, 2021. Therefore, as of that date, 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	9 Months Ended
	Sep. 30, 2021
Marketable Securities and Fair Value Measurements [Abstract]
Marketable Securities and Fair Value Measurements	​ ​ 3. Marketable Securities and Fair Value Measurements The following tables present our marketable securities by security type: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of September 30, 2021 ​ ​ ​ Gross Gross ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair (in thousands) ​ Cost ​ Gains ​ Losses ​ Value U.S. treasury securities ​ $ 10,016 ​ $ — ​ $ — ​ $ 10,016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ As of September 30, 2021 and December 31, 2020, marketable securities consisted of investments that mature 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 September 30, 2021 Using: (in thousands) ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 249,500 ​ $ — ​ $ — ​ $ 249,500 Marketable securities: ​ ​ ​ ​ U.S. treasury securities ​ — ​ 10,016 ​ — ​ 10,016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 have 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.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2021
Accrued Expenses [Abstract]
Accrued Expenses	4. Accrued Expenses Accrued expenses and other current liabilities consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2021 ​ December 31, 2020 (in thousands) ​ ​ ​ ​ ​ Accrued employee compensation and benefits ​ $ 5,163 ​ $ 6,150 Accrued external research and development expenses ​ 2,584 ​ 1,772 Accrued professional fees ​ 1,140 ​ 940 Deferred rent ​ — ​ 1,389 Other ​ 740 ​ 702 Total ​ $ 9,627 ​ $ 10,953 ​

Leases

Leases	9 Months Ended
	Sep. 30, 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 In July 2021, we entered into a 12-year 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 to scale ceDNA manufacturing utilizing RES for cGMP-compliant clinical and initial commercial supply. In addition, the new facility will house expanded capacity for research production and process development activities. Subject to certain conditions, the Manufacturing Lease is anticipated to commence in June 2022, 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 expect to account for the Manufacturing Lease in our consolidated financial statements beginning in the fourth quarter of 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 have an option to extend the Manufacturing Lease term for two additional terms of five years at the then-current fair market value. 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 Three Months Ended September 30, 2021 ​ ​ For the Nine Months Ended September 30, 2021 Operating lease cost ​ $ 1,505 ​ $ 4,514 Variable lease cost ​ ​ 454 ​ ​ 1,346 Total ​ $ 1,959 ​ $ 5,860 ​ Net cash paid for the amounts included in the measurement of the operating lease liability on the condensed consolidated balance sheet and operating activities in our condensed consolidated statement of cash flows was $5.3 million for the period ending September 30, 2021. The weighted-average remaining lease term and weighted-average incremental borrowing rate for all leases as of September 30, 2021 was 7.6 years and 6.5%, respectively. Future lease payments for our noncancelable operating leases as of September 30, 2021 and a reconciliation to the carrying amount of the operating lease liability presented in the condensed consolidated balance sheet as of September 30, 2021 are as follows: ​ ​ ​ ​ Year Ending December 31, ​ (in thousands) 2021 (remaining 3 months) ​ $ 1,842 2022 ​ 7,267 2023 ​ 7,441 2024 ​ 7,679 2025 ​ 7,881 Thereafter ​ 27,570 Total undiscounted payments due under operating leases ​ ​ 59,680 Less imputed interest ​ ​ (13,004) Total ​ $ 46,676 ​ ​ ​ ​ Current operating lease liability ​ $ 4,359 Non-current operating lease liability ​ ​ 42,317 Total ​ $ 46,676 ​

Convertible Preferred Stock

Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2021
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	6. Convertible Preferred Stock Prior to the IPO, we had 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 Convertible 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 Convertible 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 Convertible Preferred Stock. Upon the closing of the IPO in June 2020, our Convertible Preferred Stock automatically converted into 27,094,085 shares of common stock.

Equity

Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Equity	7. Equity As of September 30, 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. Common stockholders are not entitled to receive dividends, unless declared by the board of directors.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Stock incentive plans Our 2017 Stock Incentive Plan, or the 2017 Plan, provided for us to grant incentive stock options 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, 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, 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. 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 the IPO, fair value of common stock is based on quoted market prices. As of September 30, 2021, 2,862,735 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. Grant of stock options During the nine months ended September 30, 2021, we granted service-based options to certain employees, non-employees, and directors for the purchase of 1,898,989 shares of common stock with a weighted average grant date fair value of $19.51 per share that vest over a weighted average period of approximately four years. 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 number of shares of common stock reserved for issuance under the 2020 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year commencing 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, the number of shares of common stock authorized for issuance under the 2020 ESPP was increased from 481,231 shares to 950,931 shares. During the nine months ended September 30, 2021, we issued 15,511 shares of common stock under the 2020 ESPP and 935,420 shares remained available for issuance as of September 30, 2021. Stock-based compensation Stock-based compensation expense was classified in the condensed consolidated statements of operations and comprehensive loss as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ (in thousands) 2021 2020 2021 2020 Research and development expenses ​ $ 2,498 ​ $ 1,204 ​ $ 6,772 ​ $ 2,967 ​ General and administrative expenses ​ 2,472 ​ 957 ​ 6,054 ​ 2,066 ​ Total ​ $ 4,970 ​ $ 2,161 ​ $ 12,826 ​ $ 5,033 ​ ​ As of September 30, 2021, total unrecognized compensation cost related to unvested stock-based awards was $50.0 million, which is expected to be recognized over a weighted average period of 2.6 years. Additionally, as of September 30, 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.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. Commitments and Contingencies ​ ​ 401(k) Plan We have a defined-contribution plan under Section 401(k) of the Internal Revenue Code of 1986, as amended, 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 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 party to litigation arising in the ordinary course of business. We were not subject to any material legal proceedings during the nine months ended September 30, 2021.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Loss per Share	​ ​ 10. 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: ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ 2021 2020 Unvested restricted common stock 145,567 930,476 Unvested restricted common stock units ​ 14,760 ​ 26,242 Stock options to purchase common stock 6,071,008 5,581,717 Total 6,231,335 6,538,435 ​

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 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 condensed 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.
Unaudited interim financial information	Unaudited interim financial information The condensed consolidated balance sheet as of December 31, 2020 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited financial statements as of September 30, 2021 and for the three and nine months ended September 30, 2021 and 2020 have been prepared by us pursuant to the rules and regulations of the Securities and Exchange Commission, or SEC, for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These financial statements should be read in conjunction with our audited financial statements included in our Annual Report on Form 10-K that was most recently filed with the SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair presentation of our financial position as of September 30, 2021, the results of operations for the three and nine months ended September 30, 2021 and 2020, and cash flows for the nine months ended September 30, 2021 and 2020 have been made. The results of operations for the three and nine months ended September 30, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021 or any other period.
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. Our marketable securities as of September 30, 2021 consisted of U.S. government 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 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 in the event of a significant interruption in the supply process.
Marketable securities	Marketable securities Our marketable securities that consist of debt securities as of September 30, 2021 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. Effective January 1, 2021, we adopted ASU 2016-13, Financial Instruments – Credit Losses (Topic 326), using the effective date method. 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 condensed 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 condensed 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.
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 condensed 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.
Recently issued accounting pronouncements	Recently adopted accounting pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). The standard requires 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 applies the new lease requirements at the adoption date, not the earliest period presented, and recognizes 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 will not be 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 new standard. The adoption of the new standard has had a material impact on our condensed 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 condensed consolidated balance sheet of approximately $49.7 million and $33.4 million, net of deferred rent, respectively, as of January 1, 2021, which are presented as separate line items on the condensed consolidated balance sheet as of September 30, 2021. The adoption of the standard did not have a material impact on our condensed consolidated statement of operations and comprehensive loss and did not require a cumulative adjustment to accumulated deficit on our condensed consolidated statement of stockholders’ equity as of September 30, 2021. In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326). The new standard adjusts the accounting for assets held at amortized costs basis, including marketable securities accounted for as available-for-sale. The standard eliminates the probable initial recognition threshold and requires an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses is a valuation account that is 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 is 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 condensed 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 simplify 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 is effective for annual reporting periods beginning after December 15, 2021 and to interim periods within fiscal years beginning after December 15, 2022. Early adoption is 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. As an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012, we elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, we could adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard. However, as of June 30, 2021, the market value of our common stock that was held by non-affiliates exceeded $700.0 million, and as a result, we will no longer qualify as an emerging growth company as of December 31, 2021. Therefore, as of that date, we will no longer be able to take advantage of the extended transition period for adopting new or revised accounting standards.

Marketable Securities and Fai_2

Marketable Securities and Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Marketable Securities and Fair Value Measurements [Abstract]
Summary of marketable securities	The following tables present our marketable securities by security type: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of September 30, 2021 ​ ​ ​ Gross Gross ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair (in thousands) ​ Cost ​ Gains ​ Losses ​ Value U.S. treasury securities ​ $ 10,016 ​ $ — ​ $ — ​ $ 10,016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 September 30, 2021 Using: (in thousands) ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 249,500 ​ $ — ​ $ — ​ $ 249,500 Marketable securities: ​ ​ ​ ​ U.S. treasury securities ​ — ​ 10,016 ​ — ​ 10,016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Accrued Expenses [Abstract]
Summary of Accrued expenses and other current liabilities	Accrued expenses and other current liabilities consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2021 ​ December 31, 2020 (in thousands) ​ ​ ​ ​ ​ Accrued employee compensation and benefits ​ $ 5,163 ​ $ 6,150 Accrued external research and development expenses ​ 2,584 ​ 1,772 Accrued professional fees ​ 1,140 ​ 940 Deferred rent ​ — ​ 1,389 Other ​ 740 ​ 702 Total ​ $ 9,627 ​ $ 10,953

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Future lease payments for noncancelable operating lease	Future lease payments for our noncancelable operating leases as of September 30, 2021 and a reconciliation to the carrying amount of the operating lease liability presented in the condensed consolidated balance sheet as of September 30, 2021 are as follows: ​ ​ ​ ​ Year Ending December 31, ​ (in thousands) 2021 (remaining 3 months) ​ $ 1,842 2022 ​ 7,267 2023 ​ 7,441 2024 ​ 7,679 2025 ​ 7,881 Thereafter ​ 27,570 Total undiscounted payments due under operating leases ​ ​ 59,680 Less imputed interest ​ ​ (13,004) Total ​ $ 46,676 ​ ​ ​ ​ Current operating lease liability ​ $ 4,359 Non-current operating lease liability ​ ​ 42,317 Total ​ $ 46,676

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Summary of stock-based compensation expense was classified in the statements of operations and comprehensive loss	Stock-based compensation expense was classified in the condensed consolidated statements of operations and comprehensive loss as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ (in thousands) 2021 2020 2021 2020 Research and development expenses ​ $ 2,498 ​ $ 1,204 ​ $ 6,772 ​ $ 2,967 ​ General and administrative expenses ​ 2,472 ​ 957 ​ 6,054 ​ 2,066 ​ Total ​ $ 4,970 ​ $ 2,161 ​ $ 12,826 ​ $ 5,033 ​

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Summary of potential dilutive securities, presented based on amounts outstanding	​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ 2021 2020 Unvested restricted common stock 145,567 930,476 Unvested restricted common stock units ​ 14,760 ​ 26,242 Stock options to purchase common stock 6,071,008 5,581,717 Total 6,231,335 6,538,435

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional information (Detail) - USD ($) $ in Thousands	Jun. 16, 2020	Jan. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Date of incorporation					Oct. 21, 2016
Net proceeds from shares issued		$ 211,300
Net loss			$ (31,855)	$ (20,849)	$ (88,208)	$ (56,297)
Accumulated deficit			$ (277,183)		$ (277,183)		$ (188,975)
Common Stock [Member]
Shares issued during the period		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	1,578,947	1,200,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Jan. 01, 2021
Accounting Policies [Abstract]
Lease liability	$ 46,676		$ 49,700
Operating lease right-of-use assets	$ 31,252		$ 33,400
Market value threshold to qualify as an emerging growth company		$ 700,000

Marketable Securities and Fai_3

Marketable Securities and Fair Value Measurements - Summary of marketable securities (Detail) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Amortized cost		$ 199,429
Gross unrealized gains		13
Gross unrealized losses		(4)
Fair value		199,438
U.S. treasury securities
Marketable Securities [Line Items]
Amortized cost	$ 10,016	129,446
Gross unrealized gains		9
Gross unrealized losses		(4)
Fair value	$ 10,016	129,451
Commercial paper
Marketable Securities [Line Items]
Amortized cost		52,441
Fair value		52,441
Corporate debt securities
Marketable Securities [Line Items]
Amortized cost		17,542
Gross unrealized gains		4
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	Sep. 30, 2021	Dec. 31, 2020
Cash equivalents:
Total		$ 263,265
Marketable securities:
Marketable securities		199,438
Level 1
Cash equivalents:
Total		63,827
Level 2
Cash equivalents:
Total		199,438
Money Market Funds
Cash equivalents:
Cash equivalents	$ 249,500	63,827
Money Market Funds | Level 1
Cash equivalents:
Cash equivalents	249,500	63,827
U.S. treasury securities
Marketable securities:
Marketable securities	10,016	129,451
U.S. treasury securities | Level 2
Marketable securities:
Marketable securities	$ 10,016	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

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued expenses and other current liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accrued Expenses [Abstract]
Accrued employee compensation and benefits	$ 5,163	$ 6,150
Accrued external research and development expenses	2,584	1,772
Accrued professional fees	1,140	940
Deferred rent		1,389
Other	740	702
Total	$ 9,627	$ 10,953

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands	1 Months Ended	9 Months Ended
	Jul. 31, 2021USD ($)ft²	Sep. 30, 2021USD ($)	Oct. 31, 2021USD ($)
Lessee operating lease year of expiry		2029
Operating lease term	12 years	12 years
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		$ 5,300
Weighted-average incremental borrowing		6.50%
Weighted-average remaining lease term		7 years 7 months 6 days
Total rent payments		$ 59,680
Letter of Credit
Security deposit	3,600
Maximum
Estimated monthly base rent on operating lease	$ 800
Subsequent Event [Member]
Total rent payments			$ 104,300

Leases - Costs included in oper

Leases - Costs included in operating expenses related to the operating lease (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021
Leases [Abstract]
Operating lease costs	$ 1,505	$ 4,514
Variable lease cost	454	1,346
Total	$ 1,959	$ 5,860

Leases - Future lease payments

Leases - Future lease payments for noncancelable operating lease (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jan. 01, 2021
Leases [Abstract]
2021 (remaining 3 months)	$ 1,842
2022	7,267
2023	7,441
2024	7,679
2025	7,881
Thereafter	27,570
Total undiscounted payments due under operating leases	59,680
Less imputed interest	(13,004)
Current operating lease liability	4,359
Non-current operating lease liability	42,317
Total	$ 46,676	$ 49,700

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 09, 2020	Sep. 30, 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)	Jun. 16, 2020shares	Sep. 30, 2021USD ($)$ / sharesshares	Aug. 31, 2021USD ($)	Jan. 31, 2021USD ($)shares	Sep. 30, 2021$ / sharesshares	Sep. 30, 2020shares	Dec. 31, 2020$ / sharesshares
Common stock, shares authorized		150,000,000			150,000,000		150,000,000
Common stock, par value per share | $ / shares		$ 0.0001			$ 0.0001		$ 0.0001
Preferred stock, shares authorized		5,000,000			5,000,000		5,000,000
Preferred stock, par value per share | $ / shares		$ 0.0001			$ 0.0001		$ 0.0001
Net proceeds from shares issued | $				$ 211,300,000
Number of vote per common share | $		1
Maximum
Aggregate gross offering price | $			$ 250,000,000
Common Stock [Member]
Shares issued during the period				9,200,000	9,200,000	12,105,263
Common Stock [Member] | Over-Allotment Option [Member]
Shares issued during the period	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	Sep. 30, 2021	Jan. 31, 2021	Dec. 31, 2020	Jan. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock compensation on account of unvested options		$ 50
Unrecognized stock compensation on account of unvested options remaining period over which the compensation is to be recognized		2 years 7 months 6 days
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] | Certain Employees Directors and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of options granted		1,898,989
Share based compensation by share based payment arrangement number of options granted weighted average exercise price per share		$ 19.51
Period of vesting of employee stock options		4 years
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		2,862,735
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
Common stock shares available for future issuance		935,420
Issuance of common stock under other equity plans, shares		15,511
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					10,275,717	8,407,405
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

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-Based Compensation Expense was Classified in the Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 4,970	$ 2,161	$ 12,826	$ 5,033
Research and development expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	2,498	1,204	6,772	2,967
General and administrative expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 2,472	$ 957	$ 6,054	$ 2,066

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 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	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	6,231,335	6,538,435
Unvested restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	145,567	930,476
Unvested restricted common stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	14,760	26,242
Stock option to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	6,071,008	5,581,717