Generation Bio (GBIO) 10-K 18 Mar 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 28, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Interactive Data Current	Yes
Entity Central Index Key	0001733294
Current Fiscal Year End Date	--12-31
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity Registrant Name	GENERATION BIO CO.
Trading Symbol	GBIO
Entity File Number	001-39319
Entity Incorporation, State or Country Code	DE
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 Ex Transition Period	false
ICFR Auditor Attestation Flag	false
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Public Float			$ 646.2
Entity Common Stock, Shares Outstanding		56,427,399

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 62,889	$ 15,076
Marketable securities	199,438
Tenant receivable		448
Prepaid expenses and other current assets	5,408	2,577
Restricted cash		55
Total current assets	267,735	18,156
Property and equipment, net	23,781	21,845
Restricted cash, noncurrent	2,051	2,052
Deferred offering costs	336	87
Other long-term assets	252
Total assets	294,155	42,140
Current liabilities:
Accounts payable	267	2,251
Accrued expenses and other current liabilities	10,953	6,907
Total current liabilities	11,220	9,158
Deferred rent, net of current portion	14,922	15,981
Total liabilities	26,142	25,139
Commitments and contingencies (Note 11)
Convertible preferred stock (Series A, B and C), $0.0001 par value; no shares and 26,425,664 shares authorized at December 31, 2020 and 2019, respectively; no shares and 26,425,664 shares issued and outstanding at December 31, 2020 and 2019, respectively		115,593
Stockholders' equity (deficit):
Preferred stock, $0.0001 par value; 5,000,000 and no shares authorized at December 31, 2020 and 2019, respectively; no shares issued or outstanding
Common stock, $0.0001 par value; 150,000,000 and 46,750,000 shares authorized at December 31, 2020 and 2019, respectively; 46,970,012 and 6,976,077 shares issued at December 31, 2020 and 2019, respectively; 46,291,877 and 5,270,889 shares outstanding at December 31, 2020 and 2019, respectively	5	1
Additional paid-in capital	456,974	9,859
Accumulated other comprehensive income	9
Accumulated deficit	(188,975)	(108,452)
Total stockholders' equity (deficit)	268,013	(98,592)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 294,155	$ 42,140

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Convertible preferred stock, par value per share	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	26,425,664
Convertible preferred stock, shares issued	0	26,425,664
Convertible preferred stock, shares outstanding	0	26,425,664
Preferred stock, par value per share	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	0
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	46,750,000
Common stock, shares issued	46,970,012	6,976,077
Common stock, shares outstanding	46,291,877	5,270,889

Consolidated Statements of Oper

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

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series C Convertible Preferred Stock [Member]	Series A, B and C Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance, convertible preferred stock at Dec. 31, 2018			$ 115,593
Beginning balance, convertible preferred stock, shares at Dec. 31, 2018			26,425,664
Beginning balance at Dec. 31, 2018	$ (41,591)			$ 1	$ 5,552	$ (9)	$ (47,135)
Beginning balance, shares at Dec. 31, 2018				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 on marketable securities	9					9
Net loss	(61,317)						(61,317)
Ending balance, convertible preferred stock at Dec. 31, 2019	$ 115,593		$ 115,593
Ending balance, convertible preferred stock, shares at Dec. 31, 2019	26,425,664		26,425,664
Ending balance at Dec. 31, 2019	$ (98,592)			$ 1	9,859		(108,452)
Ending balance, shares at Dec. 31, 2019				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
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)
Conversion of convertible preferred stock into common stock upon initial public offering	224,425			$ 3	224,422
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 of $3,185	210,715			$ 1	210,714
Issuance of common stock upon initial public offering, net of issuance costs of $3,185, shares				12,105,263
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			792,845
Vesting of restricted common stock, shares				1,022,234
Vesting of restricted common stock units, shares				6,561
Stock-based compensation expense	$ 8,412				8,412
Unrealized gains on marketable securities	9					9
Net loss	$ (80,523)						(80,523)
Ending balance, convertible preferred stock, shares at Dec. 31, 2020	0
Ending balance at Dec. 31, 2020	$ 268,013			$ 5	$ 456,974	$ 9	$ (188,975)
Ending balance, shares at Dec. 31, 2020				46,291,877

Consolidated Statements of Co_2

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Common Stock [Member] | IPO [Member]
Stock issuance of costs	$ 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, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (80,523)	$ (61,317)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,412	4,207
Depreciation and amortization expense	3,432	1,864
Amortization (accretion) of premium (discount) on marketable securities, net	249	(317)
Gain on sale of property and equipment		(10)
Changes in operating assets and liabilities:
Tenant receivable	448	840
Prepaid expenses and other current assets	(2,831)	(1,863)
Other noncurrent assets	(252)
Accounts payable	(1,444)	761
Accrued expenses and other current liabilities	3,426	2,470
Deferred rent	(1,059)	13,019
Net cash used in operating activities	(70,142)	(40,346)
Cash flows from investing activities:
Purchases of property and equipment	(5,518)	(19,986)
Proceeds from sale of property and equipment		80
Purchases of marketable securities	(243,178)	(20,789)
Sales and maturities of marketable securities	43,500	88,680
Net cash provided by (used in) investing activities	(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 initial public offering of common stock, net of underwriting discounts and commissions	213,900
Proceeds from exercise of stock options	3,567	100
Payments of deferred offering costs	(41)	(22)
Payment of initial public offering costs	(3,185)
Net cash provided by financing activities	323,095	78
Net increase in cash, cash equivalents and restricted cash	47,757	7,717
Cash, cash equivalents and restricted cash at beginning of period	17,183	9,466
Cash, cash equivalents and restricted cash at end of period	64,940	17,183
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	494	644
Issuance costs included in accrued expenses	$ 295	$ 65

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation The Company was incorporated on October 21, 2016 as Torus Therapeutics, Inc. and subsequently changed its name to Generation Bio Co. (the “Company”). The Company is an innovative genetic medicines company creating a new class of gene therapy utilizing its proprietary non-viral non-viral The Company is 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, 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 the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. In June 2020, the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold shares of its common stock, including shares per share, resulting follow-on 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, the Company has funded its 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, and most recently, with proceeds from the sale of common stock in underwritten public offerings. The Company has incurred recurring losses, including net losses of $80.5 million for the year ended December 31, 2020. As of December 31, 2020, the Company had an accumulated deficit of $189.0 million. The Company expects to continue to generate operating losses in the foreseeable future. As of March 18, 2021, the issuance date of these consolidated financial statements, the Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months. The Company will need to obtain additional funding through public or private equity offerings, debt financings, collaborations, strategic alliances and/or licensing arrangements. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company 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 the Company’s stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or programs. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, pipeline expansion or commercialization efforts, which could adversely affect its business prospects. Although management will continue to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations when needed or at all. The accompanying consolidated financial statements reflect the operations of the Company and the Company’s 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 (“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 (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
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. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes 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 the Company to concentrations of credit risk consist primarily of cash, cash equivalents, and marketable securities. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash and cash equivalents are held. The Company maintains its cash equivalents in money market funds that invest in U.S. treasury securities. The Company’s marketable securities as of December 31, 2020 consisted of U.S government treasury securities, commercial paper and corporate debt securities. The Company has adopted an investment policy that limits the amounts the Company may invest in the securities of single issuer with the exclusion of the U.S. government. The Company has not experienced any credit losses and does not believe that it is subject to significant credit risk. The Company is dependent on a small number of third-party suppliers for its drug substance and drug product. In particular, the Company relies, and expects to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates it may develop for its programs. These programs could be adversely affected by a significant interruption in the supply process. Cash equivalents The Company considers 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 the Company’s leased facilities. These amounts are classified as restricted cash (current and noncurrent) on the Company’s consolidated balance sheets. Marketable securities The Company’s marketable securities, which consisted of debt securities as of December 31, 2020, are classified as available-for-sale and on available-for-sale debt The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Fair value measurements Certain assets and liabilities of the Company 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. The Company’s 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 the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. Deferred rent Payment escalations, rent holidays and other lease incentives that may be included in lease agreements are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the respective lease term. Adjustments for such items are recorded as deferred rent and amortized over the respective lease terms. 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 Impairment of long-lived assets Long-lived assets consist of property and equipment. The Company evaluates the recoverability of its long-lived assets when circumstances indicate that an event of impairment may have occurred. The Company recognizes 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. The Company did not record any impairment losses on long-lived assets during the years ended December 31, 2020 or 2019. Classification and accretion of convertible preferred stock The Company’s 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 the control of the Company and would have required the redemption of the then-outstanding convertible preferred stock. Segment information The Company has determined that its chief executive officer is the chief operating decision maker (“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, the Company has concluded that it operates 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 The Company has 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. The Company records accruals for costs incurred but not billed. When evaluating the adequacy of the accrued liabilities, the Company analyzes 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 the Company’s estimates. The Company’s 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 The Company measures stock options with service-based vesting or performance-based vesting granted to employees, non-employees 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. The Company uses 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. The Company accounts for forfeitures of share-based awards as they occur. The Company classifies stock-based compensation expense in its 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, 2020 and 2019, the Company’s only element of other comprehensive loss was unrealized gains (losses) on available-for-sale Income taxes The Company accounts 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 the Company’s 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. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, 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. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process deemed more-likely-than-not to Net loss per share Prior to closing of the IPO, the Company followed the two-class two-class two-class Basic net income (loss) per share attributable to common stockholders is 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 is 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 is 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. The Company’s participating securities contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Subsequent to the closing of its IPO, the Company only has 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 the Company reports 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. The Company reported a net loss attributable to common stockholders for each of the years ended December 31, 2020 and 2019. Recently adopted accounting pronouncements In August 2018, the FASB issued ASU 2018-13, 2018-13 2018-03 Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has 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, the Company can adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. In February 2016, the FASB issued ASU 2016-02, 2018-11, 2019-10, 2020-05, one-year 2016-02, of-use In June 2016, the FASB issued ASU 2016-13, available-for-sale. 2019-10, In December 2019, the FASB issued ASU No. 2019-12 ,

Marketable Securities And Fair

Marketable Securities And Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Marketable securities and fair value measurements	3. Marketable securities and fair value measurements As of December 31, 2020, marketable securities by security type consisted of the following: Gross Gross Estimated Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) U.S. treasury securities $ 129,446 $ 9 $ (4 ) $ 129,451 Commercial paper 52,441 — — 52,441 Corporate debt securities 17,542 4 — 17,546 Totals $ 199,429 $ 13 $ (4 ) $ 199,438 The Company did not have marketable securities as of December 31, 2019. The following tables present the Company’s assets that are measured at fair value on a recurring basis and their level within the fair value hierarchy, which is described further within Note 2, Fair Value Measurements at December 31, 2020 Using: Level 1 Level 2 Level 3 Total (in thousands) 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 Totals $ 63,827 $ 199,438 $ — $ 263,265 Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total (in thousands) Cash equivalents: Money market funds $ 15,076 $ — $ — $ 15,076 Totals $ 15,076 $ — $ — $ 15,076 Money market funds were valued by the Company based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. Marketable securities were valued by the Company 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, 2020
Property, Plant and Equipment [Abstract]
Property and equipment, net	4. Property and equipment, net Property and equipment, net consisted of the following: December 31, 2020 2019 (in thousands) Laboratory equipment $ 9,313 $ 5,029 Computer equipment and software 848 740 Furniture and fixtures 826 826 Leasehold improvements 17,374 12,993 Construction in progress 1,082 4,487 29,443 24,075 Less: Accumulated depreciation and amortization (5,662 ) (2,230 ) $ 23,781 $ 21,845 Depreciation and amortization expense for the years ended December 31, 2020 and 2019 was $3.4 million and $1.9 million, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	5. Accrued expenses Accrued expenses consisted of the following: December 31, 2020 2019 (in thousands) Accrued employee compensation and benefits $ 6,150 $ 3,255 Accrued external research and development expenses 1,772 1,344 Accrued professional fees 940 619 Deferred rent 1,389 1,370 Other 702 319 $ 10,953 $ 6,907

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2020
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	6. Convertible preferred stock Prior to the IPO, the Company had issued Series A convertible preferred stock (the “Series A”), Series B convertible preferred stock (the “Series B”) and Series C convertible preferred stock (the “Series C”). Collectively the Series A, Series B and Series C are referred to as the Preferred Stock. On January 9, 2020, the Company 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. The Company incurred issuance costs in connection with this transaction of $2.6 million. Upon issuance of each class of Preferred Stock, the Company assessed the embedded conversion and liquidation features of the shares and determined that such features did not require the Company to separately account for these features. The Company 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, the Company’s Preferred Stock automatically converted into 27,094,085 shares of common stock.

Equity

Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Equity	7. Equity As of December 31, 2020, the Company’s restated certificate of incorporation authorizes the Company 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. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s 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, 2020
Share-based Payment Arrangement [Abstract]
Stock-based compensation	8. Stock-based compensation The Company’s 2017 Stock Incentive Plan (the “2017 Plan”) provided for the Company to grant incentive or nonstatutory stock options, restricted stock, restricted stock units and other equity awards to employees, non-employees In May 2020 the Company’s board of directors adopted, and in June 2020 the Company’s stockholders approved, the 2020 Stock Incentive Plan (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. Upon effectiveness of the 2020 Plan, the number of shares of common stock reserved for issuance under the 2020 Plan was 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 the Company 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. Upon the effectiveness of the 2020 Plan, the Company 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 the Company’s 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 December 31, 2020, 2,500,761 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 the Company at their original issuance price pursuant to a contractual repurchase right will be available for future awards under the 2020 Plan. The number of authorized shares reserved for issuance under the 2020 Plan was increased by 1,878,800 shares effective as of January 1, 2021 in accordance with the provisions of the 2020 Plan described above. Employee stock purchase plan In May 2020 the Company’s board of directors adopted, and in June 2020 the Company’s stockholders approved, the 2020 Employee Stock Purchase Plan (the “2020 ESPP”), which became effective June 11, 2020. The 2020 ESPP will be administered by the Company’s board of directors or by a committee appointed by the board of directors. The 2020 ESPP initially provides participating employees with the opportunity to purchase up to an aggregate of 481,231 shares of common stock. 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. As of December 31, 2020, no offering periods had commenced under the 2020 ESPP and 481,231 shares were available for issuance. The number of authorized shares reserved for issuance under the 2020 ESPP was increased by 469,700 shares effective as of January 1, 2021 in accordance with the provisions of the 2020 ESPP described above. Stock option valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company completed its IPO in 2020 and, accordingly, lacks company-specific historical and implied volatility information. Therefore, it estimated its 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 it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s 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 the Company has never paid cash dividends and does 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, 2020 2019 Risk-free interest rate 0.60 % 2.00 % Expected volatility 77.30 % 75.7 % Expected dividend yield — — Expected term (in years) 6.0 6.0 The following table summarizes the Company’s stock option activity since December 31, 2019: Weighted Weighted Average Average Aggregate Number Exercise Contractual Intrinsic of Shares Price Term Value (in years) (in thousands) Outstanding as of December 31, 2019 3,373,884 $ 4.76 8.79 $ 8,492 Granted 3,090,315 13.40 Exercised (792,845 ) 4.49 Forfeited (429,584 ) 4.73 Outstanding as of December 31, 2020 5,241,770 $ 9.89 8.69 $ 99,467 Vested and expected to vest as of December 31, 2020 4,585,120 $ 10.57 8.63 $ 84,238 Options exercisable as of December 31, 2020 1,108,099 $ 4.54 7.68 $ 26,388 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 the Company’s common stock for those stock options that had strike prices lower than the fair value of the Company’s common stock. The weighted average grant-date fair value of awards granted during the years ended December 31, 2020 and 2019 was $10.41 per share and $4.46 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2020 and 2019 was $20.0 million and $0.1 million, respectively. Restricted common stock units During the year ended December 31, 2020, the Company issued 26,242 restricted stock units with a fair value of $0.5 million. The Company did not issue restricted stock units in the year ended December 31, 2019. Restricted common stock The following table summarizes the Company’s restricted common stock activity since December 31, 2019: Shares Weighted Average Unvested restricted common stock as of December 31, 2019 1,705,188 $ 1.22 Issued — — Vested (1,022,234 ) 1.17 Forfeited (4,819 ) 1.95 Unvested restricted common stock as of December 31, 2020 678,135 $ 1.28 The total fair value of restricted common stock vested during the years ended December 31, 2020 and 2019 was approximately $15.8 million and $7.0 million, respectively. Stock-Based Compensation The Company records compensation cost for all share-based payment arrangements, including employee, non-employee Year Ended December 31, 2020 2019 (in thousands) Research and development expenses $ 4,301 $ 2,753 General and administrative expenses 4,111 1,454 $ 8,412 $ 4,207 As of December 31, 2020, total unrecognized compensation cost related to unvested stock-based awards was $27.1 million, which is expected to be recognized over a weighted average period of 3.2 years. Additionally, as of December 31, 2020, the Company has unrecognized compensation cost related to unvested stock-based awards with performance-based vesting conditions for which performance has not been deemed probable of $2.2 million.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income taxes	9. Income taxes For the years ended December 31, 2020 and 2019, the Company 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 the Company’s operating losses since inception have been generated in the United States. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2020 2019 Federal statutory income tax rate (21.0 )% (21.0 )% State income taxes, net of federal benefit (7.0 ) (5.9 ) Federal and state research and development tax credits (4.2 ) (5.1 ) Stock-based compensation expense (2.6 ) 1.0 Other — 0.2 Change in deferred tax asset valuation allowance 34.8 30.8 Effective income tax rate 0.0 % 0.0 % Net deferred tax assets as of December 31, 2020 and 2019 consisted of the following: December 31, 2020 2019 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 48,280 $ 25,023 Research and development tax credit carryforwards 9,385 5,998 Deferred rent 4,456 4,740 Other 2,747 1,469 Total deferred tax assets 64,868 37,230 Deferred tax liabilities: Property and equipment (3,900 ) (4,117 ) Total deferred tax liabilities (3,900 ) (4,117 ) Valuation allowance (60,968 ) (33,113 ) Net deferred tax assets $ — $ — As of December 31, 2020, the Company had federal net operating loss carryforwards of $177.3 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 $169.1 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, 2020, the Company had state net operating loss carryforwards of $174.7 million, which may be available to offset future taxable income and expire at various dates beginning in 2036. In addition, as of December 31, 2020, the Company also had federal and state research and development tax credit carryforwards of $6.3 million and $3.9 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 (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. The Company has 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 the Company has 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 the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, The Company has evaluated the positive and negative evidence bearing upon its 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 the Company’s 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 the Company 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, 2020 and 2019. The Company reevaluates 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, 2020 and 2019 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards. The changes in the valuation allowance for 2020 and 2019 were as follows: Year Ended December 31, 2020 2019 (in thousands) Valuation allowance as of beginning of year $ 33,113 $ 14,246 Increases recorded to income tax provision 27,855 18,867 Valuation allowance as of end of year $ 60,968 $ 33,113 The Company assesses the uncertainty in its income tax positions to determine whether a tax position of the Company 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 Company files income tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company is open to future tax examination under statute from 2017 to the present. In March 2020, the Coronavirus Aid, Relief, and Economic Security Act (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 the Company’s income tax provision.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2020
License Agreements Disclosure [Abstract]
License Agreements	10. License agreements NIH The Company has 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 (“NIH”), entered into in 2017, pursuant to which NIH granted the Company a non-exclusive under certain NIH intellectual property related to the Company’s 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. The Company is 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 country-by-country basis licensed product-by-licensed product country-by-country basis last-to-expire Unless terminated earlier, the agreement remains in effect until the last to expire of the licensed patent rights on a licensed product-by-licensed country-by-country basis. During each of the years ended December 31, 2020 and 2019, the Company recorded research and development expense of less than $0.1 million under this agreement. UMass The Company has an agreement with the University of Massachusetts as represented by and solely on behalf of its Medical School (“UMass”), entered into in 2017, pursuant to which UMass granted the Company an exclusive license, with the right to grant sublicenses, under the UMass intellectual property related to the Company’s ceDNA construct. The Company is 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 country-by-country basis low-to-mid non-refundable non-creditable licensed product-by-licensed product country-by-country basis last-to-expire As part of the arrangement, the Company agreed to issue 125,677 shares of its common stock to UMass. The Company recorded $0.1 million as research and development expense and additional-paid-in Unless terminated earlier, the agreement will continue until the last-to-expire During each of the years ended December 31, 2020 and 2019, the Company recorded research and development expense of less than $0.1 million under this agreement. Other license agreements The Company has other license agreements under which it may become subject to future additional fees and milestone payments.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and contingencies The Company leases its office and laboratory space under a noncancelable operating lease that was entered into in August 2018, as amended in July 2019, and expires in 2029. The Company has an option to extend for one additional term of five years at the greater of the then-current base rent, or the then-current fair market value. The lease agreement includes provisions for a free-rent period and annual rent increases, which are accrued or deferred as appropriate such that rent expense for the lease is recognized on a straight-line basis over the lease term (see Note 2). The Company is responsible for real estate taxes, maintenance, and other operating expenses applicable to the leased premises. The lease provides for a tenant improvement allowance, at the cost of the lessor, not to exceed $14.7 million. Costs incurred by the Company for tenant improvements but not yet reimbursed by the landlord are presented on the accompanying consolidated balance sheets as tenant receivable. The Company posted a customary letter of credit in the amount of approximately $2.1 million as a security deposit. The letter of credit is subject to increase if the Company were to sublease any portion of the leased premises. Previously, the Company leased its office and laboratory space under noncancelable operating leases that expired during 2019. Rent expense for the years ended December 31, 2020 and 2019 was $7.5 million and $7.9 million, respectively. Future minimum lease payments as of December 31, 2020 are as follows: Year Ending December 31, (in thousands) 2021 $ 7,053 2022 7,251 2023 7,441 2024 7,679 2025 7,881 Thereafter 27,570 $ 64,875 401(k) Plan The Company has a defined-contribution plan under Section 401(k) of the Code (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. , beginning on January 1, 2021, Indemnification agreements In the ordinary course of business, the Company 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, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company, 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 the Company could be required to make under these indemnification agreements is, in many cases, unlimited. The Company has not incurred any material costs as a result of such indemnifications and is not currently aware of any indemnification claims. Legal proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the years ended December 31, 2020 and 2019.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss per Share	12. Net loss per share Net loss per share The Company has 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. The Company 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, 2020 2019 Convertible preferred stock (as converted to common stock) — 14,961,027 Unvested restricted common stock 678,135 1,705,188 Unvested restricted common stock units 19,681 — Stock options to purchase common stock 5,241,770 3,373,884 5,939,586 20,040,099

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information [Text Block]	13. Selected Quarterly Financial Data (Unaudited) The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information: Dec. 31, Sept. 30, June 30, March 31, Dec. 31, Sept. 30, June 30, March 31, (in thousands, except per share) Total operating expenses $ 24,345 $ 20,969 $ 17,764 $ 18,036 $ 16,158 $ 16,333 $ 15,335 $ 14,476 Loss from operations (24,345 ) (20,969 ) (17,764 ) (18,036 ) (16,158 ) (16,333 ) (15,335 ) (14,476 ) Net loss (24,226 ) (20,849 ) (17,731 ) (17,717 ) (16,063 ) (16,128 ) (15,032 ) (14,094 ) Basic and diluted net loss per share, basic and diluted $ (0.53 ) $ (0.46 ) $ (1.50 ) $ (3.22 ) $ (3.09 ) $ (3.31 ) $ (3.30 ) $ (3.29 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
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. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes 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 the Company to concentrations of credit risk consist primarily of cash, cash equivalents, and marketable securities. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash and cash equivalents are held. The Company maintains its cash equivalents in money market funds that invest in U.S. treasury securities. The Company’s marketable securities as of December 31, 2020 consisted of U.S government treasury securities, commercial paper and corporate debt securities. The Company has adopted an investment policy that limits the amounts the Company may invest in the securities of single issuer with the exclusion of the U.S. government. The Company has not experienced any credit losses and does not believe that it is subject to significant credit risk. The Company is dependent on a small number of third-party suppliers for its drug substance and drug product. In particular, the Company relies, and expects to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates it may develop for its programs. These programs could be adversely affected by a significant interruption in the supply process.
Cash equivalents	Cash equivalents The Company considers 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 the Company’s leased facilities. These amounts are classified as restricted cash (current and noncurrent) on the Company’s consolidated balance sheets.
Marketable securities	Marketable securities The Company’s marketable securities, which consisted of debt securities as of December 31, 2020, are classified as available-for-sale and on available-for-sale debt The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Fair value measurements	Fair value measurements Certain assets and liabilities of the Company 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. The Company’s 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 the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities.
Deferred rent	Deferred rent Payment escalations, rent holidays and other lease incentives that may be included in lease agreements are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the respective lease term. Adjustments for such items are recorded as deferred rent and amortized over the respective lease terms.
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
Impairment of long-lived assets	Impairment of long-lived assets Long-lived assets consist of property and equipment. The Company evaluates the recoverability of its long-lived assets when circumstances indicate that an event of impairment may have occurred. The Company recognizes 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. The Company did not record any impairment losses on long-lived assets during the years ended December 31, 2020 or 2019.
Classification and accretion of convertible preferred stock	Classification and accretion of convertible preferred stock The Company’s 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 the control of the Company and would have required the redemption of the then-outstanding convertible preferred stock.
Segment information	Segment information The Company has determined that its chief executive officer is the chief operating decision maker (“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, the Company has concluded that it operates 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 The Company has 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. The Company records accruals for costs incurred but not billed. When evaluating the adequacy of the accrued liabilities, the Company analyzes 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 the Company’s estimates. The Company’s 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 The Company measures stock options with service-based vesting or performance-based vesting granted to employees, non-employees 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. The Company uses 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. The Company accounts for forfeitures of share-based awards as they occur. The Company classifies stock-based compensation expense in its 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, 2020 and 2019, the Company’s only element of other comprehensive loss was unrealized gains (losses) on available-for-sale
Income taxes	Income taxes The Company accounts 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 the Company’s 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. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, 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. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process deemed more-likely-than-not to
Net loss per share	Net loss per share Prior to closing of the IPO, the Company followed the two-class two-class two-class Basic net income (loss) per share attributable to common stockholders is 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 is 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 is 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. The Company’s participating securities contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Subsequent to the closing of its IPO, the Company only has 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 the Company reports 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. The Company reported a net loss attributable to common stockholders for each of the years ended December 31, 2020 and 2019.
Recently issued accounting pronouncements	Recently adopted accounting pronouncements In August 2018, the FASB issued ASU 2018-13, 2018-13 2018-03 Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has 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, the Company can adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. In February 2016, the FASB issued ASU 2016-02, 2018-11, 2019-10, 2020-05, one-year 2016-02, of-use In June 2016, the FASB issued ASU 2016-13, available-for-sale. 2019-10, In December 2019, the FASB issued ASU No. 2019-12 ,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020
Fair Value Disclosures [Abstract]
Summary of marketable securities	As of December 31, 2020, marketable securities by security type consisted of the following: Gross Gross Estimated Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) U.S. treasury securities $ 129,446 $ 9 $ (4 ) $ 129,451 Commercial paper 52,441 — — 52,441 Corporate debt securities 17,542 4 — 17,546 Totals $ 199,429 $ 13 $ (4 ) $ 199,438
Summary of assets measured at fair value on a recurring basis	The following tables present the Company’s assets that are measured at fair value on a recurring basis and their level within the fair value hierarchy, which is described further within Note 2, Fair Value Measurements at December 31, 2020 Using: Level 1 Level 2 Level 3 Total (in thousands) 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 Totals $ 63,827 $ 199,438 $ — $ 263,265 Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total (in thousands) Cash equivalents: Money market funds $ 15,076 $ — $ — $ 15,076 Totals $ 15,076 $ — $ — $ 15,076

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Summary of Property and equipment, net	Property and equipment, net consisted of the following: December 31, 2020 2019 (in thousands) Laboratory equipment $ 9,313 $ 5,029 Computer equipment and software 848 740 Furniture and fixtures 826 826 Leasehold improvements 17,374 12,993 Construction in progress 1,082 4,487 29,443 24,075 Less: Accumulated depreciation and amortization (5,662 ) (2,230 ) $ 23,781 $ 21,845

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Summary of Accrued expenses and other current liabilities	Accrued expenses consisted of the following: December 31, 2020 2019 (in thousands) Accrued employee compensation and benefits $ 6,150 $ 3,255 Accrued external research and development expenses 1,772 1,344 Accrued professional fees 940 619 Deferred rent 1,389 1,370 Other 702 319 $ 10,953 $ 6,907

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
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, 2020 2019 Risk-free interest rate 0.60 % 2.00 % Expected volatility 77.30 % 75.7 % Expected dividend yield — — Expected term (in years) 6.0 6.0
Summarizes the Company's stock option activity	The following table summarizes the Company’s stock option activity since December 31, 2019: Weighted Weighted Average Average Aggregate Number Exercise Contractual Intrinsic of Shares Price Term Value (in years) (in thousands) Outstanding as of December 31, 2019 3,373,884 $ 4.76 8.79 $ 8,492 Granted 3,090,315 13.40 Exercised (792,845 ) 4.49 Forfeited (429,584 ) 4.73 Outstanding as of December 31, 2020 5,241,770 $ 9.89 8.69 $ 99,467 Vested and expected to vest as of December 31, 2020 4,585,120 $ 10.57 8.63 $ 84,238 Options exercisable as of December 31, 2020 1,108,099 $ 4.54 7.68 $ 26,388
summarizes the Company's restricted common stock activity	The following table summarizes the Company’s restricted common stock activity since December 31, 2019: Shares Weighted Average Unvested restricted common stock as of December 31, 2019 1,705,188 $ 1.22 Issued — — Vested (1,022,234 ) 1.17 Forfeited (4,819 ) 1.95 Unvested restricted common stock as of December 31, 2020 678,135 $ 1.28
Summary of stock-based compensation expense was classified in the statements of operations and comprehensive loss	The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss: Year Ended December 31, 2020 2019 (in thousands) Research and development expenses $ 4,301 $ 2,753 General and administrative expenses 4,111 1,454 $ 8,412 $ 4,207

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
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 the Company’s effective income tax rate is as follows: Year Ended December 31, 2020 2019 Federal statutory income tax rate (21.0 )% (21.0 )% State income taxes, net of federal benefit (7.0 ) (5.9 ) Federal and state research and development tax credits (4.2 ) (5.1 ) Stock-based compensation expense (2.6 ) 1.0 Other — 0.2 Change in deferred tax asset valuation allowance 34.8 30.8 Effective income tax rate 0.0 % 0.0 %
Summary of Net deferred tax assets	Net deferred tax assets as of December 31, 2020 and 2019 consisted of the following: December 31, 2020 2019 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 48,280 $ 25,023 Research and development tax credit carryforwards 9,385 5,998 Deferred rent 4,456 4,740 Other 2,747 1,469 Total deferred tax assets 64,868 37,230 Deferred tax liabilities: Property and equipment (3,900 ) (4,117 ) Total deferred tax liabilities (3,900 ) (4,117 ) Valuation allowance (60,968 ) (33,113 ) Net deferred tax assets $ — $ —
Summary of changes in the valuation allowance for deferred tax assets	The changes in the valuation allowance for deferred tax assets during the year ended December 31, 2020 and 2019 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards. The changes in the valuation allowance for 2020 and 2019 were as follows: Year Ended December 31, 2020 2019 (in thousands) Valuation allowance as of beginning of year $ 33,113 $ 14,246 Increases recorded to income tax provision 27,855 18,867 Valuation allowance as of end of year $ 60,968 $ 33,113

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Summary of future minimum lease payments	Future minimum lease payments as of December 31, 2020 are as follows: Year Ending December 31, (in thousands) 2021 $ 7,053 2022 7,251 2023 7,441 2024 7,679 2025 7,881 Thereafter 27,570 $ 64,875

Net Loss per Share (Tables)

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

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
Selected Quarterly Financial Information [Abstract]
Summary of information has been derived from unaudited consolidated financial statements	The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information: Dec. 31, Sept. 30, June 30, March 31, Dec. 31, Sept. 30, June 30, March 31, (in thousands, except per share) Total operating expenses $ 24,345 $ 20,969 $ 17,764 $ 18,036 $ 16,158 $ 16,333 $ 15,335 $ 14,476 Loss from operations (24,345 ) (20,969 ) (17,764 ) (18,036 ) (16,158 ) (16,333 ) (15,335 ) (14,476 ) Net loss (24,226 ) (20,849 ) (17,731 ) (17,717 ) (16,063 ) (16,128 ) (15,032 ) (14,094 ) Basic and diluted net loss per share, basic and diluted $ (0.53 ) $ (0.46 ) $ (1.50 ) $ (3.22 ) $ (3.09 ) $ (3.31 ) $ (3.30 ) $ (3.29 )

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Jun. 16, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Date of incorporation											Oct. 21, 2016
Net loss		$ (24,226)	$ (20,849)	$ (17,731)		$ (17,717)	$ (16,063)	$ (16,128)	$ (15,032)	$ (14,094)	$ (80,523)	$ (61,317)
Used in BS		$ (188,975)					$ (108,452)				$ (188,975)	$ (108,452)
Common Stock [Member]
Shares issued during the period											12,105,263
Common Stock [Member] | IPO [Member]
Shares issued during the period					12,105,263
Shares issued price					$ 19
Sale of Stock, Consideration Received on Transaction					$ 210,700
Common Stock [Member] | IPO [Member] | Subsequent Event [Member]
Shares issued during the period	9,200,000
Sale of Stock, Consideration Received on Transaction	$ 211,300
Common Stock [Member] | Over-Allotment Option [Member]
Shares issued during the period					1,578,947
Common Stock [Member] | Over-Allotment Option [Member] | Subsequent Event [Member]
Shares issued during the period	1,200,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary Of Property And Equipment (Detail)	12 Months Ended
	Dec. 31, 2020
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	5 years
Computer Equipment And Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	3 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	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 Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Accounting Policies [Line Items]
Income Tax rate settlement	50.00%
Operating Lease, Right-of-Use Asset	$ 49.7
Operating Lease, Liability	$ 33.4
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OperatingLeaseLiabilityCurrent

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, 2020	Dec. 31, 2019
Marketable securities:
Marketable securities	$ 199,438
Totals	263,265	$ 15,076
Level 1
Marketable securities:
Totals	63,827	15,076
Level 2
Marketable securities:
Totals	199,438
Money Market Funds
Cash equivalents:
Cash equivalents	63,827	15,076
Money Market Funds | Level 1
Cash equivalents:
Cash equivalents	63,827	$ 15,076
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, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 29,443	$ 24,075
Less: Accumulated depreciation and amortization	(5,662)	(2,230)
Property, Plant and Equipment, Net	23,781	21,845
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	9,313	5,029
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	848	740
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	826	826
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	17,374	12,993
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,082	$ 4,487

Property and Equipment, Net - A

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

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Expenses [Line Items]
Accrued employee compensation and benefits	$ 6,150	$ 3,255
Accrued external research and development expenses	1,772	1,344
Accrued professional fees	940	619
Deferred rent	1,389	1,370
Other	702	319
Total	$ 10,953	$ 6,907

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	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	Dec. 31, 2020	Dec. 31, 2019
Equity [Abstract]
Common stock, shares authorized	150,000,000	46,750,000
Common stock, par value per share	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	0
Preferred stock, par value per share	$ 0.0001	$ 0.0001

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jun. 11, 2020	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2021	Jan. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compenation by share based payment arrangement number of options granted		3,090,315
Unrecognised stock compensation on account of unvested options		$ 27.1
Unrecognised stock compensation on account of unvested options remaining period over which the compensation is to be recognised		3 years 2 months 12 days
weighted average grant-date fair value		$ 1.17
Equity Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
weighted average grant-date fair value		$ 10.41	$ 4.46
No of Instrinic stock option exercised		$ 20	$ 0.1
Unvested Stock Awards for Which Performance is not Probable
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognised stock compensation on account of unvested options		$ 2.2
Service Based Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compenation by share based payment arrangement number of options granted		26,242	0
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		$ 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		2,500,761
2020 Stock Incentive Plan | Maximum | Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance				1,878,800
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]
Share based compensation by share based payment arrangement number of shares authorized for issuance	481,231
Common stock shares available for future issuance		481,231
Employee Stock Purchase Plan 2020 | Maximum | Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares available for future issuance				469,700
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

Stock-based compensation - Summ

Stock-based compensation - Summary of Inputs Used in Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Risk-free interest rate	0.60%	2.00%
Expected volatility	77.30%	75.70%
Expected term (in years)	6 years	6 years

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, 2020	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Number of shares Outstanding , Beginning	3,373,884
Number of shares | Granted	3,090,315
Number of shares | Exercised	(792,845)
Number of shares | Forfeited	(429,584)
Number of shares Outstanding , Ending	5,241,770	3,373,884
Number of shares | Vested and expected to vest	4,585,120
Number of shares | Options exercisable	1,108,099
Weighted average exercise price Outstanding , Beginning	$ 4.76
Weighted average exercise price | Granted	13.40
Weighted average exercise price | Exercised	4.49
Weighted average exercise price | Forfeited	4.73
Weighted average exercise price Outstanding , Ending	9.89	$ 4.76
Weighted average exercise price | Vested and expected to vest	10.57
Weighted average exercise price | Options exercisable	$ 4.54
Weighted average contractual term (in years) Outstanding	8 years 8 months 8 days	8 years 9 months 14 days
Weighted average contractual term (in years) | Vested and expected to vest	8 years 7 months 17 days
Weighted average contractual term (in years) | Options exercisable	7 years 8 months 4 days
Aggregate intrinsic value (in thousands) Outstanding	$ 99,467	$ 8,492
Aggregate intrinsic value (in thousands) | Vested and expected to vest	84,238
Aggregate intrinsic value (in thousands) | Options exercisable	$ 26,388

Stock-based compensation - Su_3

Stock-based compensation - Summarizes the Company's Restricted Common Stock Activity (Detail)	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Share-based Payment Arrangement [Abstract]
Unvested restricted common stock, Beginning | Shares | shares	1,705,188
Vested | Shares | shares	(1,022,234)
Forfeited | Shares | shares	(4,819)
Unvested restricted common stock, Ending | Shares | shares	678,135
Weighted average grant date fair value of Unvested restricted common stock, Beginning | $ / shares | $ / shares	$ 1.22
Weighted average grant date fair value of Unvested restricted common stock, Vested | $ / shares | $ / shares	1.17
Weighted average grant date fair value of Unvested restricted common stock, Forfeited | $ / shares | $ / shares	1.95
Weighted average grant date fair value of Unvested restricted common stock, Ending | $ / shares | $ / shares	$ 1.28

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, 2020	Dec. 31, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 8,412	$ 4,207
Research and development expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	4,301	2,753
General and administrative expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 4,111	$ 1,454

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
IncomeTax [Line Items]
Net operating loss carry forwards	$ 169,100								$ 169,100
Percentage on annual taxable income									80.00%
Tax Credit Carryforward, Amount	3,900								$ 3,900
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
Total net operations loss	24,345	$ 20,969	$ 17,764	$ 18,036	$ 16,158	$ 16,333	$ 15,335	$ 14,476	$ 81,114	$ 62,302
Federal income tax Authority [Member]
IncomeTax [Line Items]
Net operating loss carry forwards	177,300								177,300
State and Local Jurisdiction [Member]
IncomeTax [Line Items]
Net operating loss carry forwards	174,700								174,700
Federal and state research and development tax credit carry forwards [Member]
IncomeTax [Line Items]
Tax Credit Carryforward, Amount	$ 6,300								$ 6,300

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, 2020	Dec. 31, 2019
Valuation Allowance [Abstract]
Valuation allowance as of beginning of year	$ 33,113	$ 14,246
Increases recorded to income tax provision	27,855	18,867
Valuation allowance as of end of year	$ 60,968	$ 33,113

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 48,280	$ 25,023
Research and development tax credit carryforwards	9,385	5,998
Deferred rent	4,456	4,740
Other	2,747	1,469
Total deferred tax assets	64,868	37,230
Deferred tax liabilities:
Property and equipment	(3,900)	(4,117)
Total deferred tax liabilities	(3,900)	(4,117)
Valuation allowance	$ (60,968)	$ (33,113)	$ (14,246)

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, 2020	Dec. 31, 2019
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	(21.00%)	(21.00%)
State income taxes, net of federal benefit	(7.00%)	(5.90%)
Federal and state research and development tax credits	(4.20%)	(5.10%)
Stock-based compensation expense	(2.60%)	1.00%
Other		0.20%
Change in deferred tax asset valuation allowance	34.80%	30.80%
Effective income tax rate	0.00%	0.00%

License Agreements - Additional

License Agreements - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
License Agreements Disclosure [Line Items]
Research and development	$ 58,532	$ 50,134
NIH [Member]
License Agreements Disclosure [Line Items]
Research and development	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
Adjustments to Additional Paid in Capital, Other	$ 100

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of Commitement and Contingencies [Line Items]
Lessee operating lease year of expiry	2029
Operating lease agreement option to renewal term	5 years
Tenant improvement allowance maximum	$ 14.7
Letter of Credit for Security Deposit	2.1
Rent expense operating lease	$ 7.5	$ 7.9
Lease existence of option to extend	true
Defined contribution plan, percentage	4.00%

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments (Detail) $ in Thousands	Dec. 31, 2020USD ($)
Commitments and Contingencies Disclosure [Abstract]
2021	$ 7,053
2022	7,251
2023	7,441
2024	7,679
2025	7,881
Thereafter	27,570
Future Minimum Lease Payments	$ 64,875

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, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	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
Unvested restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	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	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	5,241,770	3,373,884

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Summary of information has been derived from unaudited consolidated financial statements (Detail) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Selected Quarterly Financial Information [Abstract]
Total operating expenses	$ 24,345	$ 20,969	$ 17,764	$ 18,036	$ 16,158	$ 16,333	$ 15,335	$ 14,476	$ 81,114	$ 62,302
Loss from operations	(24,345)	(20,969)	(17,764)	(18,036)	(16,158)	(16,333)	(15,335)	(14,476)	(81,114)	(62,302)
Net loss	$ (24,226)	$ (20,849)	$ (17,731)	$ (17,717)	$ (16,063)	$ (16,128)	$ (15,032)	$ (14,094)	$ (80,523)	$ (61,317)
Basic and diluted net loss per share, basic and diluted	$ (0.53)	$ (0.46)	$ (1.50)	$ (3.22)	$ (3.09)	$ (3.31)	$ (3.30)	$ (3.29)	$ (2.95)	$ (12.96)