Evelo Biosciences (EVLO) 10-K 9 Mar 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Mar. 05, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-38473
Entity Registrant Name	Evelo Biosciences, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-5594527
Entity Address, Address Line One	620 Memorial Drive
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	577-0300
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	EVLO
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Public Float			$ 112.8
Entity Common Stock, Shares Outstanding		53,334,947
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement for its 2021 annual meeting of stockholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant’s fiscal year ended December 31, 2020, are incorporated by reference into Part III of this Annual Report on Form 10-K.
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001694665

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 68,857	$ 77,833
Prepaid expenses and other current assets	2,123	3,176
Total current assets	70,980	81,009
Property and equipment, net	7,478	8,341
Right of use asset - operating lease	10,757
Other assets	1,424	1,570
Total assets	90,639	90,920
Current liabilities:
Accounts payable	1,442	620
Accrued expenses	16,254	8,758
Operating lease liability, current portion	1,674
Other current liabilities	463	365
Total current liabilities	19,833	9,743
Noncurrent liabilities:
Long-term debt	30,048	19,634
Operating lease liability, net of current portion	9,989
Deferred rent, net of current portion		1,148
Other noncurrent liabilities	284	198
Total liabilities	60,154	30,723
Commitments and contingencies (Note 9)
Stockholder’s equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued and outstanding at December 31, 2020 and 2019, respectively	0	0
Common stock, $0.001 par value; 200,000,000 shares authorized; 47,488,505 and 32,232,258 shares issued and 47,470,119 and 32,170,605 shares outstanding at December 31, 2020 and 2019, respectively	47	32
Additional paid-in capital	322,957	259,018
Accumulated deficit	(292,519)	(198,853)
Total stockholders’ equity	30,485	60,197
Total liabilities and stockholders’ equity	$ 90,639	$ 90,920

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	47,488,505	32,232,258
Common stock, shares outstanding (in shares)	47,470,119	32,170,605

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	$ 69,616	$ 63,128
General and administrative	22,270	23,229
Total operating expenses	91,886	86,357
Loss from operations	(91,886)	(86,357)
Other (expense) income:
Interest (expense) income, net	(2,109)	1,049
Other income, net	738	26
Other (expense) income, net	(1,371)	1,075
Loss before income taxes	(93,257)	(85,282)
Income tax expense	(409)	(190)
Net loss	$ (93,666)	$ (85,472)
Weighted-average number of common shares outstanding, basic and diluted (in shares)	39,479,197	32,031,862
Net loss per share, basic and diluted (in dollars per share)	$ (2.37)	$ (2.67)
Comprehensive loss:
Net loss	$ (93,666)	$ (85,472)
Other comprehensive loss:
Unrealized gain on investments, net of tax of $0	0	18
Comprehensive loss	$ (93,666)	$ (85,454)

Consolidated Statements of Op_2

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Tax portion of unrealized loss on investments	$ 0	$ 0

Consolidated Statements of Stoc

Consolidated Statements of Stockholders’ Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2018		31,825,769
Beginning balance at Dec. 31, 2018	$ 136,949	$ 32	$ 250,316	$ (18)	$ (113,381)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)		64,118
Vesting of restricted common stock	26		26
Exercise of stock options (in shares)		280,718
Exercise of stock options	511		511
Stock-based compensation expense	8,165		8,165
Unrealized gain on investments	18			18
Net loss	(85,472)				(85,472)
Ending balance (in shares) at Dec. 31, 2019		32,170,605
Ending balance at Dec. 31, 2019	60,197	$ 32	259,018	0	(198,853)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of fees (in shares)		15,032,131
Issuance of common stock, net of fees	54,994	$ 15	54,979
Vesting of restricted common stock (in shares)		43,267
Vesting of restricted common stock	21		21
Issuance of common stock under the Employee Stock Purchase Plan (in shares)		28,603
Issuance of common stock under the Employee Stock Purchase Plan	$ 92		92
Exercise of stock options (in shares)	195,513	195,513
Exercise of stock options	$ 379		379
Stock-based compensation expense	8,468		8,468
Unrealized gain on investments	0
Net loss	(93,666)				(93,666)
Ending balance (in shares) at Dec. 31, 2020		47,470,119
Ending balance at Dec. 31, 2020	$ 30,485	$ 47	$ 322,957	$ 0	$ (292,519)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating activities
Net loss	$ (93,666)	$ (85,472)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,468	8,165
Depreciation expense	2,026	1,764
Net accretion of discount on marketable securities	0	(164)
Non-cash interest expense	374	255
Non-cash lease expense	1,976
Gain on sale of fixed assets	(6)	(2)
Prepaid expenses and other current assets	1,503	372
Accounts payable	837	(585)
Accrued expenses and other current liabilities	7,438	3,694
Operating lease liabilities	(2,218)
Other liabilities	205	(7)
Net cash used in operating activities	(73,063)	(71,980)
Investing activities
Proceeds from sales and maturities of investments	0	55,000
Purchases of property and equipment	(1,321)	(3,032)
Proceeds from the sale of fixed assets	6	2
Net cash (used in) provided by investing activities	(1,315)	51,970
Financing activities
Net proceeds from the issuance of common stock, net of issuance cost	54,994	0
Net proceeds from the issuance of long-term debt	10,000	19,481
Proceeds from issuance of common stock under employee stock purchase plan and the exercise of stock options, restricted common stock	471	511
Repayment of long-term debt	0	(15,000)
Net cash provided by financing activities	65,465	4,992
Net increase in cash, cash equivalents and restricted cash	(8,913)	(15,018)
Cash, cash equivalents and restricted cash – beginning of year	79,333	94,351
Cash, cash equivalents and restricted cash – end of year	70,420	79,333
Supplemental disclosure of cash flow information
Cash paid for interest	2,172	1,166
Cash paid for taxes	20	0
Noncash investing and financing activities
Property and equipment additions in accounts payable and accrued expenses	178	246
Public offering cost in accrued expenses	$ 111	$ 0

Organization and Basis of Prese

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Basis of Presentation	Organization and Basis of Presentation Evelo Biosciences, Inc. ("Evelo" or the "Company”) is a biotechnology company which was incorporated in Delaware on May 6, 2014. The Company is discovering and developing oral biologics that act on cells in the small intestine with systemic therapeutic effects. The Company is advancing these oral biologics with the aim of treating a broad range of immune mediated diseases with an initial focus on inflammatory diseases and oncology. The Company is headquartered in Cambridge, Massachusetts. Since inception, the Company has devoted substantially all of its efforts to research and development and raising capital. The Company has not generated any revenue related to its primary business purpose to date. The Company is subject to a number of risks similar to those of other development stage companies, including dependence on key individuals, the need to develop commercially viable products, competition from other companies, many of whom are larger and better capitalized, and the need to obtain adequate additional financing to fund the development of its products. To date, the Company has financed operations primarily with the proceeds from issuance of common stock combined with proceeds from previous sales of convertible preferred stock to equity investors and debt financing. On June 3, 2019, the Company filed a Registration Statement on Form S-3 (File No. 333-231911) (the “Shelf”) with the SEC in relation to the registration of common stock, preferred stock, debt securities, warrants and/or units of any combination thereof in the aggregate amount of up to $200.0 million for a period of up to three years from the date of its effectiveness on June 6, 2019. The Company also simultaneously entered into a sales agreement (the "ATM") with Cowen and Company, LLC, as sales agent, providing for the offering, issuance and sale by the Company of up to an aggregate $50.0 million of its common stock from time to time in “at-the-market” offerings under the Shelf. For the year ended December 31, 2020, the Company sold 1,232,131 common shares under the ATM with offering prices ranging between $4.25 to $11.15 per share for gross proceeds of $6.8 million and net proceeds of $6.6 million, after deducting commission and other offering expenses payable by us. In January 2021, the Company issued 139,734 additional shares of common stock under the ATM with offering prices ranging between $12.54 and $13.17 per share for gross proceeds of $1.8 million and net proceeds of $1.7 million, after deducting commission and other offering expenses. In June 2020, the Company sold 13,800,000 shares of its common stock in an underwritten public offering at a public offering price of $3.75 per share, including the underwriters' exercise of their option to purchase 1,800,000 shares to cover over-allotment, generating gross proceeds of $51.8 million and net proceeds of $48.4 million, after deducting underwriting discounts and commission and other offering expenses payable by the Company. On July 14, 2020, the Company drew down the second tranche of $10.0 million available under the 2019 Credit Facility. Refer to Note 7, Loan and Security Agreement, to this Annual Report on Form 10-K for more information. On February 2, 2021, the Company sold 5,175,000 shares of its common stock in an underwritten public offering at a public offering price of $15.00 per share, including the underwriters' exercise of their option to purchase 675,000 shares to cover over-allotment, generating gross proceeds of $77.6 million and net proceeds of underwriting discounts and commission of $73.0 million, exclusive of other offering expenses payable by the Company. On January 28, 2021, the Company entered into a stock purchase agreement with ALJ Health Care & Life Science Company Limited ("ALJ"), pursuant to which on February 2, 2021, ALJ purchased $7.5 million of our common stock in a private placement at a purchase price of $15.00 per share, equal to the public offering price per share at which our common stock was sold to the public as referred above. The sale of such shares will not be registered under the Securities Act. In accordance with the Financial Accounting Standards Board ("FASB") Accounting Standards Update ("ASU") 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. The Company has incurred recurring losses since its inception, including net losses of $93.7 million and $85.5 million for the years ended December 31, 2020 and 2019, respectively. In addition, as of December 31, 2020, the Company had an accumulated deficit of $292.5 million. The Company expects to continue to generate operating losses for the foreseeable future. The Company previously identified conditions and events that raised substantial doubt about its ability to continue as a going concern. During the first quarter of 2021 we raised net proceeds of $82.2 million from the issuance of common stock exclusive of certain other fees payable by us. The Company expects that its cash and cash equivalents as of December 31, 2020 of $68.9 million together with the net proceeds raised from the issuances of common stock in the first quarter of 2021, will be sufficient to fund the operating expenditures and capital expenditure requirements necessary to advance its research efforts and clinical trials for at least one year from the date of issuance of these consolidated financial statements. The future viability of the Company beyond one year from the date of issuance of these consolidated financial statements is dependent on its ability to raise additional capital to finance its operations. The Company's inability to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies. There can be no assurance that the current operating plan will be achieved or that additional funding will be available on terms acceptable to the Company, or at all. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standard Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuation of stock-based awards. The Company bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results could differ from those estimates. Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned, controlled subsidiaries. All intercompany transactions and balances have been eliminated in consolidation. Subsequent Event Considerations The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other than those disclosed in Note 16, Subsequent Event, to this Annual Report on Form 10-K. Emerging Growth Company Status Evelo is an “emerging growth company,” as defined in the JOBS Act, and it may take advantage of reduced reporting requirements that are otherwise applicable to public companies. Evelo may take advantage of these exemptions until it is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. Evelo has elected to use the extended transition period for complying with new or revised accounting standards; and as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. Evelo may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its IPO or such earlier time that it is no longer an emerging growth company. Evelo would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue; it has more than $700.0 million in market value of its stock held by non-affiliates (and has been a public company for at least 12 months and has filed one annual report on Form 10-K), or it has issued more than $1.0 billion of non-convertible debt securities over a three-year period. Concentrations of Credit Risk and Off-Balance Sheet Risk Financial instruments that potentially expose the Company to concentrations of credit risk primarily consist of cash and cash equivalents. The Company places its cash and cash equivalents in primarily two custodian accounts at accredited financial institutions. Such deposits have and will continue to exceed federally insured limits. As of December 31, 2020, and 2019, the Company has no off-balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. The Company is subject to a number of risks similar to other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical testing or clinical trials, its reliance on third parties to conduct its clinical trials, the need to obtain regulatory and marketing approvals for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, protection of proprietary technology, the ability to make milestone, royalty or other payments due under any license or collaboration agreements, and the need to secure and maintain adequate manufacturing arrangements with third parties. If the Company does not successfully commercialize or partner any of its product candidates, it will be unable to generate product revenue or achieve profitability. Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ equity that are excluded from net loss. The Company's only element of other comprehensive loss is unrealized gains on available-for-sale investments. For the year ended December 31, 2020 comprehensive loss was equal to net loss. Comprehensive loss totaled $85.5 million for the years ended December 31, 2019, and was not significantly different than net loss. Cash, Cash Equivalents and Restricted Cash Cash equivalents are comprised of highly liquid investments that are readily convertible into cash with original maturities of three months or less. Cash and cash equivalents include cash held in banks and amounts held in money market funds. The Company’s restricted cash consists of restricted cash in connection with a lease for the Company’s office and laboratory premises and deposits held in relation to the Company's credit card facility. As of December 31, 2020 the Company had $0.3 million in current restricted cash within prepaid expenses and other current assets in the consolidated balance sheet. The Company had no current restricted cash at December 31, 2019. As of December 31, 2020 and 2019, the Company had noncurrent restricted cash of $1.3 million and $1.5 million, respectively, which were included within other assets in the consolidated balance sheets. The following reconciles cash, cash equivalents and restricted cash as of December 31, 2020 and 2019, as presented on the Company's consolidated statements of cash flows, to its related consolidated balance sheet accounts (in thousands): December 31, 2020 2019 Cash and cash equivalents: Cash $ 4,487 $ 1,634 Money market funds 64,370 76,199 Total cash and cash equivalents 68,857 77,833 Restricted cash 1,563 1,500 Cash, cash equivalents and restricted cash $ 70,420 $ 79,333 Fair Value of Financial Instruments ASC 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances. ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following: • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly; and • Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. An entity may choose to measure many financial instruments and certain other items at fair value at specified election dates. Subsequent unrealized gains and losses on items for which the fair value option has been elected will be reported in earnings. The Company did not elect to measure any additional financial instruments or other items at fair value. Property and Equipment Property and equipment consists of computer hardware and software, furniture and fixtures, office equipment, research and lab equipment, and leasehold improvement recorded at cost. Lab equipment used in research and development activities is only capitalized when it has an alternative future use. These amounts are depreciated using the straight-line method over the estimated useful lives of the assets. Purchased assets that are not yet in service are recorded to construction-in-process and no depreciation expense is recorded. Once they are placed in service they are reclassified to the appropriate asset class. A summary of the estimated useful lives is as follows: Classification Estimated Useful Life Computer hardware 3 - 5 years Computer software 3 years Furniture and fixtures 7 years Research and lab equipment (used/new) 3/5 years Leasehold improvements Lesser of asset life or Repairs and maintenance costs are expensed as incurred. Impairment of Long-Lived Assets The Company periodically evaluates property and equipment for impairment whenever events or changes in circumstances indicate that a potential impairment may have occurred. If such events or changes in circumstances arise, the Company compares the carrying amount of the long-lived assets to the estimated future undiscounted cash flows expected to be generated by the long-lived assets. If the estimated aggregate undiscounted cash flows are less than the carrying amount of the long-lived assets, an impairment charge, calculated as the amount by which the carrying amount of the assets exceeds the fair value of the assets, is recorded. The fair value of the long-lived assets is determined based on the estimated discounted cash flows expected to be generated from the long-lived assets. The Company has not recorded any material impairment charges during the years presented. Research and Development Costs Research and development costs are expensed in the period incurred. Research and development expenses consist of both internal and external costs such as payroll, consulting, and manufacturing costs associated with the development of the Company’s product candidates. Costs for certain development activities, such as clinical trials and manufacturing development activities, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, and information provided to the Company by its vendors on their actual costs incurred or level of effort expended. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as prepaid or accrued research and development expenses. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed. The Company has and may continue to acquire the rights to develop and commercialize new product candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately recognized as research and development expense provided that there is no alternative future use of the rights in other research and development projects. Any milestone payments made for Intellectual Property after regulatory approval, or that have alternative future use, are capitalized and amortized. Income Taxes The Company records deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities and for loss and credit carryforwards using enacted tax rates expected to be in effect in the years in which the differences reverse. A valuation allowance is provided to reduce the net deferred tax assets to the amount that will more likely than not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. Stock-Based Compensation The Company records stock-based compensation for options granted to employees and directors based on the grant date fair value of awards issued. The expense is recorded over the requisite service period, which is the vesting period, on a straight-line basis. The Company uses the Black-Scholes option-pricing model to determine the fair value of stock options. The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the Company’s common stock price, as well as a number of other assumptions. The Company records forfeitures as they occur. The Company accounts for stock-based compensation arrangements with non-employees based upon the fair value of the consideration received or the equity instruments issued, whichever is more reliably measurable. The measurement date for non-employee awards is generally the date performance of services required from the non-employee is complete. Stock-based compensation costs for non-employee awards are recognized as services are provided, which is generally the vesting period, on a straight-line basis. Prior to January 1, 2020, we accounted for these awards in accordance with the provisions of ASC Subtopic 505-50, Equity-Based Payments to Non-employees (“ASC 505-50”). Under ASC 505-50, share-based awards to nonemployees were subject to periodic fair value re-measurement at the end of each financial reporting period prior to completion of the service. As discussed in below under the heading “New Accounting Pronouncements - Adopted during the current period,” the Company adopted ASU No. 2018-07, Stock-based Compensation: Improvements to Nonemployee Share-based Payment Accounting (Topic 718), on January 1, 2020. As a result, the Company’s accounting for nonemployee awards is now generally consistent with that of employee awards. Beginning on January 1, 2020, the measurement date for nonemployee awards is the date of grant without any subsequent changes in the fair value of the award. Segments The Company has one operating segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purpose of allocating resources. Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period. For purposes of the dilutive net loss per share applicable to common stockholders calculation stock options, common stock from Employee Stock Purchase Plan (the “ESPP”) and unvested restricted stock are considered to be common stock equivalents but are excluded from the calculation of diluted net loss per share applicable to common stockholders, as their effect would be anti-dilutive; therefore, basic and diluted net loss per share applicable to common stockholders were the same for all periods presented. New Accounting Pronouncements Adopted during the current period Leases In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (“ASU 2016-02”), which supersedes the guidance in former ASC 840, Leases. The new accounting guidance requires recognition of all long-term lease assets and lease liabilities by lessees and sets forth new disclosure requirements for those lease assets and liabilities. It requires lessees to recognize right-of-use assets and lease liabilities on the balance sheet using a modified retrospective approach at the beginning of the earliest comparative period in the financial statements for all leases with a term of greater than 12 months regardless of classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. The FASB subsequently issued several ASUs amending the new standard. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2018 for most public entities. The Company adopted this new standard on January 1, 2020 using the required modified retrospective approach and utilizing the effective date as its date of initial application. As a result, prior periods are presented in accordance with the previous guidance in ASC 840. ASU 2016-02 provides a number of optional practical expedients in transition. The Company elected to adopt the 'package of practical expedients', which permits the Company (i) not to reassess whether expired existing contracts are or contain leases, (ii) not to reassess the classification of expired or existing leases, and (iii) not to reassess initial direct costs for any existing leases. The Company will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. Adoption of this standard resulted in the recognition of a right-of-use asset and a lease liability on the Company’s January 1, 2020 consolidated balance sheet of $12.7 million and $13.9 million, respectively. There was no material impact resulting from the adoption on the Company’s consolidated statement of operations for the year ended December 31, 2020. For leases with terms greater than 12 months, the Company records the related right-of-use asset and lease liability at the present value of lease payments over the term. As the Company’s leases do not provide readily determinable implicit interest rates, the Company utilized its incremental borrowing rate, which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. In transition to ASC 842, the Company utilized the remaining lease term of its leases in determining the appropriate incremental borrowing rates. The application of the new standard required netting of unamortized balance of lease incentives and deferred lease obligation to the right-of-use asset at the adoption date. The Company’s operating leases include rental escalation clauses that are factored into the determination of lease payments when appropriate. The Company does not separate lease and non-lease components of contracts. Refer to Note 3, Leases, to this Annual Report on Form 10-K for additional information. Share-Based Compensation In June 2018, the FASB issued ASU No. 2018-07, Stock-based Compensation: Improvements to Nonemployee Share-based Payment Accounting (Topic 718) ("ASU 2018-07"), which amends the existing accounting standards for share-based payments to nonemployees. This ASU aligns much of the guidance on measuring and classifying nonemployee awards with that of awards to employees. Under the new guidance, the measurement of nonemployee equity awards is fixed on the grant date. Entities will apply the ASU by recognizing a cumulative-effect adjustment, if any, to retained earnings as of the beginning of the annual period of adoption. The Company adopted ASU 2018-07 on January 1, 2020. The adoption of this standard did not have a material impact to this Annual Report on Form 10-K. To be adopted in future periods In December 2019, the FASB issued ASU No. 2019 -12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The new standard includes several provisions which simplify accounting for income taxes by removing certain exceptions to the general principles in Topic 740 and increasing consistency and clarity for the users of financial statements. This standard will be effective for the Company on January 1, 2021. Early adoption is permitted. The adoption of this guidance is not expected to have a material impact on the Company’s financial position and results of operations. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326)—Measurement of Credit Losses on Financial Instruments, which has been subsequently amended by ASU No. 2018-19, ASU No. 2019-04, ASU No. 2019-05, ASU No. 2019-10, ASU No. 2019-11 and ASU No. 2020-03 (“ASU 2016-13”). The provisions of ASU 2016-13 modify the impairment model to utilize an expected loss methodology in place of the currently used incurred loss methodology and require a consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 is effective for the Company on January 1, 2023, with early adoption permitted. The Company is currently evaluating the potential impact that this standard may have on its financial position and results of operations, as well as the timing of its adoption of this standard. On August 5, 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (“ASU-2020-06”), which simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. ASU 2020-06 eliminates the beneficial conversion and cash conversion accounting models in ASC 470-20 that require separate accounting for embedded conversion features from convertible instruments. As a result, after adopting the ASU’s guidance, entities will not separately present in equity an embedded conversion feature in such debt. Additionally, the guidance simplifies the evaluation of whether a contract in the issuer’s own equity can be classified in equity or an embedded feature qualifies for the derivative scope exception. Although the guidance is not effective until 2022, early adoption of ASU 2020-06 is permitted for all entities for fiscal years beginning after December 15, 2020. The Company is currently evaluating the impact of this new guidance on the Company’s consolidated financial statements and related disclosures.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	Leases In January 2018, the Company entered into an operating sublease arrangement to lease approximately 40,765 square feet for its office and research development space at 620 Memorial Drive, Cambridge, MA 02139 from February 2018 to September 2025. The Company maintained an additional separate operating lease for office and laboratory space that expired in May 2020. The leases require security deposits, which the Company has primarily met with letters of credit from a financial institution that is secured with cash on deposit. In June 2018, the Company entered into a sublease arrangement with a third party to lease space subject to an operating lease that expired in April 2020. The minimum rental payments received under this agreement totaled $0.2 million for the year ended December 31, 2020 and were equivalent to the minimum payments due from the Company to the landlord. The Company recorded rent expense of $2.9 million for both years ended December 31, 2020 and 2019, which are net of sublease rental income of $0.3 million and $0.5 million. Sublease rental income is inclusive of rental payments, taxes, and operating expenses. The minimum aggregate future lease commitments at December 31, 2020, are as follows (in thousands): Amount 2021 $ 2,727 2022 3,062 2023 3,154 2024 3,249 2025 2,491 Total lease payments 14,683 Less imputed interest (3,020) Total $ 11,663 Other information: Operating cash flows used for operating leases $ 3,334 Weighted-average remaining lease term (in years) 5 years Weighted-average discount rate 9.5 % Under the prior lease accounting guidance, minimum rental commitments under non-cancelable leases as of December 31, 2019 were as follows (in thousands): Amount 2021 $ 2,973 2022 3,062 2023 3,154 2024 3,249 2025 2,492 Total lease payments $ 14,930

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value as of December 31, 2020 and 2019 (in thousands): Description December 31, 2020 Assets: Money market funds included within cash and cash equivalents $ 64,370 $ 64,370 $ — $ — Total $ 64,370 $ 64,370 $ — $ — Description December 31, 2019 Assets: Money market funds included within cash and cash equivalents $ 76,199 $ 76,199 $ — $ — Total $ 76,199 $ 76,199 $ — $ — As of December 31, 2020 and 2019, the Company's cash equivalents have been initially valued at the transaction price and subsequently valued utilizing a third-party pricing service. The Company validates the prices provided by its third-party pricing service by understanding the models used and obtaining market values from other pricing sources.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment consists of the following (in thousands): December 31, 2020 2019 Property and equipment: Lab equipment $ 8,831 $ 7,479 Leasehold improvements 2,157 2,014 Furniture and fixtures 822 750 Computers and software 230 204 Office equipment 3 9 Construction-in-process 1,078 1,594 Property and equipment 13,121 12,050 Less: accumulated depreciation (5,643) (3,709) Property and equipment, net $ 7,478 $ 8,341 The Company recognized $2.0 million and $1.8 million of depreciation expense for the years ended December 31, 2020 and 2019, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): December 31, 2020 2019 Accrued external research and development expenses $ 9,394 $ 4,583 Accrued payroll and related expenses 5,620 3,149 Accrued professional fees 604 659 Accrued other 636 367 Total accrued expenses $ 16,254 $ 8,758

Loan and Security Agreement

Loan and Security Agreement	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Loan and Security Agreement	Loan and Security Agreement 2016 Credit Facility In 2016, the Company entered into a credit facility (the “2016 Credit Facility”) with a bank that allowed the Company to borrow up to $15.0 million. Borrowings under the 2016 Credit Facility were secured by a lien on all Company assets, excluding intellectual property. The Company borrowed the entire $15.0 million available under the 2016 Credit Facility prior to its extinguishment in July 2019 as discussed in further detail below. The 2016 Credit Facility contained negative covenants restricting the Company’s activities, including limitations on cash deposits, dispositions, mergers or acquisitions, incurring indebtedness or liens, paying dividends or making investments and certain other business transactions. There were no financial covenants associated with the agreement. 2019 Credit Facility On July 19, 2019, the Company entered into a loan and security agreement (as amended, the "2019 Credit Facility") with K2 HealthVentures LLC and others (collectively, "K2HV") pursuant to which the K2HV agreed to make term loans in an aggregate principal amount of up to $45.0 million available to the Company in three tranches. The initial tranche of $20.0 million was funded upon closing on July 19, 2019. As amended on May 15, 2020, the second tranche of $10.0 million was available to be funded between December 1, 2019 and July 15, 2020 and was drawn down on July 14, 2020. The third tranche of $15.0 million expired on January 15, 2021. Borrowings under the 2019 Credit Facility are collateralized by substantially all of the Company's personal property, excluding intellectual property, and the Company pledged its equity interests in its subsidiaries, subject to certain limitations with respect to its foreign subsidiaries. Interest on the outstanding loan balance will accrue at a variable annual rate equal to the greater of (i) 8.65% and (ii) the prime rate plus 3.15%. The Company is required to make interest-only payments on the loans on a monthly basis through February 28, 2022. Subsequent to the interest only periods, the Company is required to make equal monthly payments of principal plus interest until the loans mature on August 1, 2024. Upon final payment or prepayment of the loans, the Company must pay a final payment equal to 4.3% of the loans borrowed, which is being accrued to interest expense over the term of the loan using the effective-interest method. The Company incurred fees associated with establishing the 2019 Credit Facility of $0.4 million. The Company has an option to prepay the loans in whole, subject to a prepayment fee of 2% of the amount prepaid or, if the prepayment occurs after the 18-month anniversary of the funding date of the loans, 1% of the amount prepaid. The 2019 Credit Facility contains customary representations, warranties and covenants and also includes customary events of default, including payment defaults, breaches of covenants, change of control and occurrence of a material adverse effect. The Company has determined that the risk of subjective acceleration under the material adverse events clause was remote and therefore has classified the long-term portion of the outstanding principal in non-current liabilities. Upon the occurrence and continuation of an event of default, a default interest rate of an additional 5% per annum may be applied to the outstanding loan balances, and the administrative agent, collateral agent, and lenders may declare all outstanding obligations immediately due and payable and exercise all of their rights and remedies as set forth in the 2019 Credit Facility and under applicable law. As of December 31, 2020, the Company was in compliance with all covenants under the 2019 Credit Facility. The Company used the proceeds from the initial $20.0 million tranche to prepay on July 19, 2019 the full $15.0 million loan balance outstanding under the 2016 Credit Facility . The Company has the following minimum aggregate future loan payments at December 31, 2020 (in thousands): Amount 2021 $ 2,631 2022 11,603 2023 13,387 2024 10,259 Total minimum payments $ 37,880 Less amounts representing interest and discount (7,832) Long-term debt $ 30,048 Interest expense related to the Company's 2016 Credit Facility was approximately $0.5 million, for the year ended December 31, 2019.

In-License Agreements

In-License Agreements	12 Months Ended
	Dec. 31, 2020
Research and Development [Abstract]
In-License Agreements	In-License Agreements Mayo Foundation for Medical Education and Research On June 10, 2016, the Company entered into a Research and License Agreement, (the “2016 Mayo License Agreement”) with the Mayo Foundation for Medical Education and Research, an affiliate of Mayo Clinic (the “Mayo Clinic”). Under the 2016 Mayo License Agreement, the Mayo Clinic was entitled to certain participation rights in connection with the issuance and sale of preferred stock that was issued prior to the Company’s public offering and warrants which were issued in 2016 and exercised in 2018. On August 6, 2017, the Company and the Mayo Clinic entered into a license agreement (“2017 Mayo License Agreement”). Under the 2017 Mayo License Agreement, the Mayo Clinic granted the Company (i) an exclusive, worldwide, sublicensable license under the Mayo Clinic’s rights to certain intellectual property and microbial strains and (ii) a non-exclusive, worldwide, sublicensable license to certain related know-how, in each case, to develop and commercialize certain microbial strains and licensed products incorporating any such strains. As consideration, the Company paid a nonrefundable upfront fee of $0.2 million and will pay annual license maintenance fees. Nonrefundable upfront fees were expensed in full to research and development expense in 2017. Annual maintenance fees will be expensed as incurred over the term of the agreement. The Company may owe the Mayo Clinic milestone payments upon the achievement of certain development, regulatory, and commercial milestones, up to a maximum of $56.0 million in the aggregate, as well as royalties on net sales of licensed products in low single-digit percentages. As of December 31, 2020, the Company has incurred milestone payments to date totaling approximately $0.2 million under the agreement of which no amounts are currently due. University of Chicago On March 10, 2016, the Company and the University of Chicago entered into a patent license agreement (“2016 University of Chicago Agreement”). Under the 2016 University of Chicago Agreement, the University of Chicago granted the Company (i) an exclusive, royalty-bearing and sublicensable license under the Licensed Patents and (ii) a non-exclusive, royalty-bearing, sublicensable license to access the technical information to diligently develop and commercialize Licensed Products. As consideration, the Company paid a nonrefundable upfront fee of less than $0.5 million and will pay annual license maintenance fees. Nonrefundable upfront fees were expensed in full to research and development expense in 2016. Annual maintenance fees will be expensed as incurred over the term of the agreement. The Company may owe the University of Chicago milestone payments, totaling an aggregate of approximately $60.9 million, upon the achievement of certain development, regulatory, and commercial milestones, as well as royalties on net sales of licensed products ranging from low to high single-digit percentages. As of December 31, 2020, the Company has incurred milestone payments to date totaling approximately $0.4 million under the agreement of which no amounts are currently due.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Collaboration Agreement with Sacco S.r.l. In July 2019, the Company entered into an agreement with Sacco S.r.l. ("Sacco"), an affiliate of one of the Company’s existing contract manufacturing organizations, pursuant to which and subject to certain exceptions for pre-existing products for pre-existing customers, Sacco will manufacture and supply single strain, non-genetically modified microbes intended for oral delivery or oral use in pharmaceutical products exclusively for the Company for a period of five years. Sacco may terminate the agreement if the provision of manufacturing services has been, or is scheduled to be, inactive for a period of six ny has incurred annual exclusivity fees to date totaling approximately €1.2 million, and no amounts are currently due as of t he year ended December 31, 2020. Agreement with Biose Industrie On February 15, 2018, the Company entered into an agreement with Biose Industrie (“Biose”), a French corporation, in which Biose agreed to exclusively manufacture certain microbial biotherapeutic products for the Company and reserved agreed upon manufacturing resources to conduct manufacturing runs for such products. Under the terms of this agreement, the Company agreed to annual fees in the mid-six digits in consideration of both exclusivity for the manufacture of those microbial biotherapeutics and for a set minimum number of manufacturing runs per year. Excl usivity fees paid and any minimum commitments are expensed as incurred. At December 31, 2020, aggregate minimum payments over the remaining contract life total approximately $0.7 million. The agreement expired on February 15, 2021 in accordance with its terms. Litigation and Other Proceedings The Company may periodically become subject to legal proceedings and claims arising in connection with on-going business activities, including claims or disputes related to patents that have been issued or that are pending in the field of research on which the Company is focused. The Company is not a party to any material litigation and does not have contingency reserves established for any litigation liabilities. On February 12, 2021, the European Patent Office issued a Communication of a Notice of Opposition for European patent EP 3223834, which is held by the Company. The Company is currently evaluating its available options and deciding next steps with respect to this matter. The patent at issue does not relate to any of the Company’s current product candidates, and receipt of this communication and/or any subsequent proceeding is not expected to affect any of the Company’s current development plans.

Stockholders_ Equity

Stockholders’ Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Stockholders’ Equity	Stockholders’ Equity Common Stock On June 3, 2019, the Company filed a Shelf with the SEC in relation to the registration of common stock, preferred stock, debt securities, warrants and/or units of any combination thereof in the aggregate amount of up to $200.0 million for a period of up to three years from the date of the filing. The Company also simultaneously entered into the ATM, providing for the offering, issuance and sale by the Company of up to an aggregate $50.0 million of its common stock from time to time in “at-the-market” offerings under the Shelf. For the year ended December 31, 2020, pursuant to the ATM, the Company sold 1,232,131 shares of its common stock, with offering prices ranging between $4.25 to $11.15 per share for gross proceeds of $6.8 million and net proceeds of $6.6 million, after deducting commission and other offering expenses payable by us. In June 2020, the Company sold 13,800,000 shares of its common stock pursuant to the Shelf in an underwritten public offering at a public offering price of $3.75 per share, for gross proceeds of $51.8 million and net proceeds of $48.4 million, after deducting underwriting discounts and commission and other offering expenses payable by the Company.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based Compensation 2018 Incentive Award Plan The Company’s board of directors adopted on April 18, 2018, and the Company’s stockholders approved, the 2018 Incentive Award Plan (the “2018 Plan”), which became effective May 8, 2018 and under which the Company may grant cash and equity-based incentive awards to the Company’s employees, officers, directors, consultants and advisors. Following the effectiveness of the 2018 Plan, the Company ceased making grants under the 2015 Stock Incentive Plan (as amended the “2015 Plan"). The 2018 Plan initially allowed the Company to grant awards for up to 1,344,692 shares of common stock plus that number of shares of common stock subject to awards outstanding under the 2015 Plan, that are forfeited, lapse unexercised or are settled in cash. Each year starting with 2019, the number of shares available for grants of awards under the 2018 Plan will be increased automatically on January 1 by a number of shares of common stock equal to the lesser of 4% of the shares of common stock outstanding on the final day of the preceding calendar year or the number of shares determined by the Company’s board of directors. Accordingly, on January 1, 2021, 2020 and 2019 the number of shares authorized for issuance under the 2018 Incentive Plan was increased by 1,898,805 shares, 1,286,824 shares and 1,273,031 shares, respectively. The 2015 Plan continues to govern the terms and conditions of the outstanding awards granted under it. The exercise price of stock options granted under the 2018 Plan is equal to not less than the fair market value of a share of the Company’s common stock on the grant date. Other terms of awards, including vesting requirements, are determined by the board of directors and are subject to the provisions of the 2018 Plan. Stock options granted to employees generally vest over a four-year period but may be granted with different vesting terms. Certain options provide for accelerated vesting in the event of a change in control. Awards granted to non-employee consultants generally vest monthly over a period of one covering 4,376,182 options of the Company’s common stock and 284,000 restricted stock units have been issued under the 2018 Plan, of which 875,155 options have been canceled and 4,640 options have been exercised. As of December 31, 2020, 951,621 shares of common stock are available for future grant under the 2018 Plan, which includes 832,101 shares subject to awards that were originally granted, and have since the effective date of the 2018 Plan been canceled or repurchased, under the 2015 Plan. 2015 Stock Incentive Plan Prior to the approval of the 2018 Plan, the Company granted equity awards under the 2015 Plan, which originally provided for grant of incentive stock options, non-qualified stock options, restricted stock awards, or RSAs, and other stock-based awards to the Company’s employees, officers, directors, consultants and advisors. The terms of equity award agreements, including vesting requirements, were determined by the board of directors and are subject to the provisions of the 2015 Plan. Stock options granted to employees generally vest over a four-year period but may be granted with different vesting terms. A limited number of awards contain performance-based vesting criteria and for such awards that are deemed probable of vesting, the Company records expense in the period in which such determination is made through any estimated remaining vesting period. Certain options provide for accelerated vesting in the event of a change in control. Awards granted to non-employee consultants generally vest monthly over a period of one Under the 2015 Plan, the Company was authorized to grant equity awards up to an aggregate of 5,417,044 shares of common stock. As of December 31, 2020, an aggregate of 5,758,518 options and other equity awards had been granted under the 2015 Plan, of which 1,376,141 have been exercised, 1,268,110 have been canceled and 18,468 have been repurchased as of December 31, 2020. A total of 113,006 shares previously reserved under the 2015 Plan that had not been exercised or were otherwise subject to outstanding exercise awards were no longer authorized as of May 8, 2018. Stock-Based Compensation Expense Stock-based compensation expense included in the Company’s statements of operations is as follows (in thousands): Year Ended December 31, 2020 2019 Research and development $ 4,487 $ 3,648 General and administrative 3,981 4,517 Total stock-based compensation expense $ 8,468 $ 8,165 Stock Options A summary of the Company’s stock option activity and related information is as follows: Shares Weighted Weighted Aggregate Options outstanding at December 31, 2019 5,691,474 $ 6.99 Granted 2,161,356 $ 6.06 Exercised (195,513) $ 1.94 Canceled (1,046,655) $ 8.80 Options outstanding at December 31, 2020 6,610,662 $ 6.55 7.53 $ 38,815 Exercisable at December 31, 2020 3,671,175 $ 5.52 6.77 $ 25,332 Vested and expected to vest as of December 31, 2020 6,610,662 $ 6.55 7.53 $ 38,815 (1) The aggregate intrinsic value of 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 exercise prices lower than the fair value of the common stock as of the end of the period. The Company had 2,957,873 unvested stock options outstanding as of December 31, 2020. The weighted-average fair value of options granted during the years ended December 31, 2020 and 2019 was $4.14 and $7.46 , respectively. The aggregate intrinsic value of options exercised during the years ended December 31, 2020 and 2019 was $0.9 million and $1.8 million, respectively. When utilizing the Black-Scholes option-pricing model to determine the grant date fair value of stock options granted to employees or non-employees, the Company used the following weighted average, or ranges of, assumptions: Employee option grants Year Ended December 31, 2020 2019 Risk-free interest rate 1.11 % 2.28 % Expected life (in years) 6.05 6.02 Volatility 79.6 % 76.2 % Expected dividend rate 0.00 % 0.00 % Expected Term: The expected term represents the period that the options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). The expected life is applied to the stock option grant group as a whole as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. Expected Volatility: The Company used an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry that were deemed to be representative of future stock price trends as the Company does not have any trading history for its common stock. Risk-Free Interest Rate: The Company based the risk-free interest rate over the expected term of the options based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant. Expected Dividend: The Company has not paid and does not anticipate paying any dividends in the near future. Therefore, the expected dividend yield was zero. Non-employee option grants Year Ended December 31, 2020 2019 Risk-free interest rate 0.38 % 1.98 % Expected life (in years) 5.21 7.63 Volatility 78.9 % 76.0 % Expected dividend rate 0.00 % 0.00 % The Company estimates the expected life of options granted based on the remaining contractual term of the option for options granted to non-employees. As of December 31, 2020, total unrecognized stock-based compensation expense relating to unvested stock options was $14.4 million. This amount is subject to change as the unvested portion of the stock options granted to non-employees is subject to re-measurement over the vesting period. This amount is expected to be recognized over a weighted average period of 2.12 years. On November 4, 2020, 284,000 RSUs were granted to certain employees of the Company under the 2018 Plan with a weighted average grant date fair value of $4.41. Each award of RSUs vests as to 25% on the first anniversary of the grant date, 25% of the RSUs on the second anniversary of the grant date and 50% of the RSUs on the third anniversary of the grant date, subject to the grantees continuing service. As of December 31, 2020, none of the restricted stock units had vested. Stock-based compensation expense related to RSUs was immaterial for the year ended December 31, 2020. 2018 Employee Stock Purchase Plan The Company's board of directors adopted on April 18, 2018, and the Company’s stockholders approved, the ESPP, which became effective on May 8, 2018. A total of 336,356 shares of common stock were initially reserved for issuance under the ESPP. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2020 and ending in 2028, by an amount equal to the lesser of (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) an amount determined by the Company’s board of directors. The Company’s board of directors determined not to increase the number of shares that may be issued under the ESPP on January 1, 2020. The Company's board of directors has authorized an initial offering period under the ESPP commencing on February 1, 2020. Accordingly, on January 1, 2021, the number of shares authorized for issuance under the ESPP was increased by 474,701 shares. The compensation expense recognized related to the ESPP for the year ended December 31, 2020 was $0.1 million. There was a total of 28,603 shares purchased under the ESPP during the year ended December 31, 2020.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company has recorded a tax provision of $0.4 million and $0.2 million for the year ended December 31, 2020 and 2019, respectively. The Company did not record a tax benefit for the periods presented due to the losses incurred and the need for a full valuation allowance on net deferred tax assets. The tax expense recorded for the December 31, 2020 and 2019 period primarily relates to current tax expense at the Company's UK subsidiary. The difference between the income tax expense at the U.S. federal statutory rate and the recorded provision is primarily due to the valuation allowance provided on all deferred tax assets. The Company’s loss before income tax for the periods presented was generated in the United States with a small profit generated by the Company's subsidiary in the United Kingdom. December 31, 2020 2019 U.S. federal tax statutory rate 21.0 % 21.0 % State taxes, net of federal benefit 6.8 % 7.0 % Non-deductible stock compensation (1.0) % (0.6) % Other non-deductible expenses (0.4) % (0.4) % Credits 1.8 % 1.6 % Change in valuation allowance (28.6) % (29.1) % Other — % 0.3 % Total (0.4) % (0.2) % December 31, 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 36,256 $ 25,895 Research and development credits 7,092 4,856 Capitalized research and development, patent and start-up costs 34,452 22,101 Accrued expenses 1,370 1,006 Stock based compensation 3,443 2,335 Operating lease liability 3,186 — Right of use asset - operating lease (2,939) — Depreciation (208) (295) Deferred tax assets before valuation allowance 82,652 55,898 Valuation allowance (82,652) (55,898) Net deferred tax assets $ — $ — As of December 31, 2020, the Company had approximately $133.7 million and $129.4 million of Federal and state Net Operating Losses (“NOLs”), respectively. The Federal NOLs include $49.9 million which expire at various dates through 2037, and $83.8 million which carryforward indefinitely. The state NOLs expire at various dates through 2040. As of December 31, 2020, the Company had federal and state research credits of $5.0 million and $2.6 million, respectively, which expire at various dates through 2040. Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. Under the Code, certain substantial changes in the Company’s ownership, including the sale of the Company or significant changes in ownership due to sales of equity, have limited and may limit in the future, the amount of net operating loss carryforwards which could be used annually to offset future taxable income. The Company has not yet completed an analysis of ownership changes. The Company may also experience ownership changes in the future as a result of subsequent shifts in our stock ownership, some of which may be outside the Company’s control. As a result, the Company’s ability to use our pre-change NOLs to offset U.S. federal taxable income may be subject to limitations, which could potentially result in increased future tax liability to the Company. In addition, at the state level, there may be periods during which the use of NOLs is suspended or otherwise limited, which could accelerate or permanently increase state taxes owed. All Federal NOLs generated post tax reform will have an indefinite life, are not subject to carryback provisions and limited to 80% of income in any year after 2020. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the net deferred tax assets as of December 31, 2020 and 2019, respectively. The valuation allowance increased by $26.8 million in 2020 primarily due to increases in net operating losses and research and development credits. As of December 31, 2020 and 2019, the Company had no unrecognized tax benefits, respectively. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense. The Company does not expect any significant change in its uncertain tax positions in the next twelve months.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share Basic and diluted net loss per common share is determined by dividing net loss attributable to common stockholders by the weighted-average common shares outstanding during the period. The Company has computed diluted net loss per common share after giving consideration to all potentially dilutive common shares, including options to purchase common stock, common stock from the ESPP and restricted common stock, outstanding during the period determined using the treasury stock methods, except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti-dilutive and therefore basic and diluted net loss per share have been equivalent. The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive: Year Ended December 31, 2020 2019 Unvested common stock from early exercise of options 18,386 61,653 Stock options to purchase common stock 6,610,662 5,691,474 RSUs 284,000 — Common stock from the ESPP 24,508 — Total 6,937,556 5,753,127

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions The C ompany receives clinical advisory services from Weatherden Ltd. (“Weatherden”) under agreements that were entered into during 2017 and 2018. Duncan McHale, the Company’s Chief Medical Officer is a part owner of Weat herden. During the years ended December 31, 2020 and 2019, the Company paid Weatherden $0.6 million and $1.0 million, respectively. As of December 31, 2020 an immaterial amount was due to Weatherden. As of December 31, 2019, the amount due to Weatherden under the supply of service agreement totaled approximately $0.2 million. In June 2018, the Company entered into a subleasing arrangement with Ring Therapeutics, Inc. (formerly VL46, Inc.), an affiliate of one of its stockholders, Flagship Venture Funds. Under the terms of the sublease, the Company invoiced Ring Therapeutics for an aggregate $0.9 million in rent payments which were due during the period from July 1, 2018 through April 30, 2020, the sublease expiration date, plus related taxes and lease operating costs. For the year ended December 31, 2020, $0.3 million related to this sublease, inclusive of rent payments, taxes and operating expenses, has been recorded as an offset to operating expense within the consolidated statements of operations and comprehensive loss. The Company entered into a consulting agreement with David Epstein (as amended, the "Consulting Agreement"), the Company's Chairman of the Board, effective September 16, 2019 pursuant to which Mr. Epstein will provide strategic advisory and other consulting services to the Company. As amended on October 15, 2020, the Agreement will continue until June 30, 2021 unless terminated earlier by either Mr. Epstein or the Company upon 30 days’ notice, or 24 hours’ notice by the non-breaching party in the event of a breach. In accordance with the terms of the Consulting Agreement, on September 16, 2019, Mr. Epstein was granted an option to purchase 75,000 shares of the Company’s common stock, which award vests in 36 equal monthly installments subject to his continued provision of consulting services to the Company pursuant to the Consulting Agreement on the applicable vesting dates. Under the Consulting Agreement, Mr. Epstein also is entitled to receive (i) an annual equity award on each anniversary of the effective date of the Consulting Agreement in the form of an option to purchase shares of the Company’s common stock having an aggregate grant date fair market value equal to approximately $0.2 million, as determined by the Board in its discretion based on customary option pricing methodologies, which award vests in 12 equal monthly installments following the grant date, subject to his continued provision of consulting services to the Company pursuant to the Consulting Agreement on the applicable vesting date, and (ii) an aggregate annual cash consulting fee of $0.3 million for his consulting services. All of the foregoing options, to the extent then outstanding, will be subject to accelerated vesting upon the occurrence of a change in control of the Company. On October 11, 2020, in connection with the commencement of his second year of service as a consultant to the Company, Mr. Epstein was granted an annual equity award in the form of an option to purchase 44,743 shares of the Company’s common stock, which award vests in nine equal monthly installments, in each case subject to his continued provision of consulting services to the Company pursuant to the Consulting Agreement on the applicable vesting dates.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution PlanThe Company provides benefits under certain retirement benefit plans. The Company's most significant defined contribution plan is in the United States, which is administered through a third-party administrator. Under the U.S. defined contribution plan employees may elect to defer up to 85.0% of their compensation per year (subject to a maximum limit prescribed by federal tax law) and the Company matches a portion of such employee contributions. For the years ended December 31, 2020 and 2019 the Company’s matching contribution expense totaled $0.3 million and $0.2 million, respectively.

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event In January 2021, pursuant to the ATM, the Company issued 139,734 shares of its common stock in an “at-the-market” offering under the Company’s previously filed Shelf, with offering prices ranging between $12.54 and $13.17 per share for gross proceeds of $1.8 million and net proceeds of $1.7 million, after deducting commission and other offering expenses payable by the Company. On February 2, 2021, the Company sold 5,175,000 shares of its common stock in an underwritten public offering at a public offering price of $15.00 per share, including the underwriters' exercise of their option to purchase 675,000 shares to cover over-allotment, generating gross proceeds of $77.6 million and net proceeds of underwriting discounts and commission of $73.0 million, exclusive of other offering expenses payable by the Company. On January 28, 2021, the Company entered into a stock purchase agreement with ALJ, pursuant to which on February 2, 2021, ALJ purchased $7.5 million of our common stock in a private placement at a purchase price of $15.00 per share, equal to the public offering price per share at which our common stock was sold to the public as referred above. The sale of such shares will not be registered under the Securities Act.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Organization and Basis of Presentation	The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standard Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).
Use of Estimates	The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuation of stock-based awards. The Company bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Principles of Consolidation	The consolidated financial statements include the accounts of the Company and its wholly owned, controlled subsidiaries. All intercompany transactions and balances have been eliminated in consolidation.
Subsequent Event Considerations	The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other than those disclosed in Note 16, Subsequent Event, to this Annual Report on Form 10-K.
Emerging Growth Company Status	Evelo is an “emerging growth company,” as defined in the JOBS Act, and it may take advantage of reduced reporting requirements that are otherwise applicable to public companies. Evelo may take advantage of these exemptions until it is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. Evelo has elected to use the extended transition period for complying with new or revised accounting standards; and as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. Evelo may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its IPO or such earlier time that it is no longer an emerging growth company. Evelo would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue; it has more than $700.0 million in market value of its stock held by non-affiliates (and has been a public company for at least 12 months and has filed one annual report on Form 10-K), or it has issued more than $1.0 billion of non-convertible debt securities over a three-year period.
Concentrations of Credit Risk and Off-Balance Sheet Risk	Financial instruments that potentially expose the Company to concentrations of credit risk primarily consist of cash and cash equivalents. The Company places its cash and cash equivalents in primarily two custodian accounts at accredited financial institutions. Such deposits have and will continue to exceed federally insured limits. As of December 31, 2020, and 2019, the Company has no off-balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. The Company is subject to a number of risks similar to other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical testing or clinical trials, its reliance on third parties to conduct its clinical trials, the need to obtain regulatory and marketing approvals for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, protection of proprietary technology, the ability to make milestone, royalty or other payments due under any license or collaboration agreements, and the need to secure and maintain adequate manufacturing arrangements with third parties. If the Company does not successfully commercialize or partner any of its product candidates, it will be unable to generate product revenue or achieve profitability.
Comprehensive Loss	Comprehensive loss includes net loss and certain changes in stockholders’ equity that are excluded from net loss. The Company's only element of other comprehensive loss is unrealized gains on available-for-sale investments.
Cash, Cash Equivalents and Restricted Cash	Cash equivalents are comprised of highly liquid investments that are readily convertible into cash with original maturities of three months or less. Cash and cash equivalents include cash held in banks and amounts held in money market funds. The Company’s restricted cash consists of restricted cash in connection with a lease for the Company’s office and laboratory premises and deposits held in relation to the Company's credit card facility. As of December 31, 2020 the Company had $0.3 million in current restricted cash within prepaid expenses and other current assets in the consolidated balance sheet. The Company had no current restricted cash at December 31, 2019. As of December 31, 2020 and 2019, the Company had noncurrent restricted cash of $1.3 million and $1.5 million, respectively, which were included within other assets in the consolidated balance sheets.
Fair Value of Financial Instruments	ASC 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances. ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following: • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly; and • Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. An entity may choose to measure many financial instruments and certain other items at fair value at specified election dates. Subsequent unrealized gains and losses on items for which the fair value option has been elected will be reported in earnings. The Company did not elect to measure any additional financial instruments or other items at fair value.
Property and Equipment	Property and equipment consists of computer hardware and software, furniture and fixtures, office equipment, research and lab equipment, and leasehold improvement recorded at cost. Lab equipment used in research and development activities is only capitalized when it has an alternative future use. These amounts are depreciated using the straight-line method over the estimated useful lives of the assets. Purchased assets that are not yet in service are recorded to construction-in-process and no depreciation expense is recorded. Once they are placed in service they are reclassified to the appropriate asset class. A summary of the estimated useful lives is as follows: Classification Estimated Useful Life Computer hardware 3 - 5 years Computer software 3 years Furniture and fixtures 7 years Research and lab equipment (used/new) 3/5 years Leasehold improvements Lesser of asset life or Repairs and maintenance costs are expensed as incurred.
Impairment of Long-Lived Assets	The Company periodically evaluates property and equipment for impairment whenever events or changes in circumstances indicate that a potential impairment may have occurred. If such events or changes in circumstances arise, the Company compares the carrying amount of the long-lived assets to the estimated future undiscounted cash flows expected to be generated by the long-lived assets. If the estimated aggregate undiscounted cash flows are less than the carrying amount of the long-lived assets, an impairment charge, calculated as the amount by which the carrying amount of the assets exceeds the fair value of the assets, is recorded. The fair value of the long-lived assets is determined based on the estimated discounted cash flows expected to be generated from the long-lived assets. The Company has not recorded any material impairment charges during the years presented.
Research and Development Costs	Research and development costs are expensed in the period incurred. Research and development expenses consist of both internal and external costs such as payroll, consulting, and manufacturing costs associated with the development of the Company’s product candidates. Costs for certain development activities, such as clinical trials and manufacturing development activities, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, and information provided to the Company by its vendors on their actual costs incurred or level of effort expended. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as prepaid or accrued research and development expenses. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed. The Company has and may continue to acquire the rights to develop and commercialize new product candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately recognized as research and development expense provided that there is no alternative future use of the rights in other research and development projects. Any milestone payments made for Intellectual Property after regulatory approval, or that have alternative future use, are capitalized and amortized.
Income Taxes	The Company records deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities and for loss and credit carryforwards using enacted tax rates expected to be in effect in the years in which the differences reverse. A valuation allowance is provided to reduce the net deferred tax assets to the amount that will more likely than not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes.
Stock-Based Compensation	The Company records stock-based compensation for options granted to employees and directors based on the grant date fair value of awards issued. The expense is recorded over the requisite service period, which is the vesting period, on a straight-line basis. The Company uses the Black-Scholes option-pricing model to determine the fair value of stock options. The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the Company’s common stock price, as well as a number of other assumptions. The Company records forfeitures as they occur. The Company accounts for stock-based compensation arrangements with non-employees based upon the fair value of the consideration received or the equity instruments issued, whichever is more reliably measurable. The measurement date for non-employee awards is generally the date performance of services required from the non-employee is complete. Stock-based compensation costs for non-employee awards are recognized as services are provided, which is generally the vesting period, on a straight-line basis. Prior to January 1, 2020, we accounted for these awards in accordance with the provisions of ASC Subtopic 505-50, Equity-Based Payments to Non-employees (“ASC 505-50”). Under ASC 505-50, share-based awards to nonemployees were subject to periodic fair value re-measurement at the end of each financial reporting period prior to completion of the service. As discussed in below under the heading “New Accounting Pronouncements - Adopted during the current period,” the Company adopted ASU No. 2018-07, Stock-based Compensation: Improvements to Nonemployee Share-based Payment Accounting (Topic 718), on January 1, 2020. As a result, the Company’s accounting for nonemployee awards is now generally consistent with that of employee awards. Beginning on January 1, 2020, the measurement date for nonemployee awards is the date of grant without any subsequent changes in the fair value of the award.
Segments	The Company has one operating segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purpose of allocating resources.
Net Loss per Share	Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period. For purposes of the dilutive net loss per share applicable to common stockholders calculation stock options, common stock from Employee Stock Purchase Plan (the “ESPP”) and unvested restricted stock are considered to be common stock equivalents but are excluded from the calculation of diluted net loss per share applicable to common stockholders, as their effect would be anti-dilutive; therefore, basic and diluted net loss per share applicable to common stockholders were the same for all periods presented.
New Accounting Pronouncements	Adopted during the current period Leases In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (“ASU 2016-02”), which supersedes the guidance in former ASC 840, Leases. The new accounting guidance requires recognition of all long-term lease assets and lease liabilities by lessees and sets forth new disclosure requirements for those lease assets and liabilities. It requires lessees to recognize right-of-use assets and lease liabilities on the balance sheet using a modified retrospective approach at the beginning of the earliest comparative period in the financial statements for all leases with a term of greater than 12 months regardless of classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. The FASB subsequently issued several ASUs amending the new standard. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2018 for most public entities. The Company adopted this new standard on January 1, 2020 using the required modified retrospective approach and utilizing the effective date as its date of initial application. As a result, prior periods are presented in accordance with the previous guidance in ASC 840. ASU 2016-02 provides a number of optional practical expedients in transition. The Company elected to adopt the 'package of practical expedients', which permits the Company (i) not to reassess whether expired existing contracts are or contain leases, (ii) not to reassess the classification of expired or existing leases, and (iii) not to reassess initial direct costs for any existing leases. The Company will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. Adoption of this standard resulted in the recognition of a right-of-use asset and a lease liability on the Company’s January 1, 2020 consolidated balance sheet of $12.7 million and $13.9 million, respectively. There was no material impact resulting from the adoption on the Company’s consolidated statement of operations for the year ended December 31, 2020. For leases with terms greater than 12 months, the Company records the related right-of-use asset and lease liability at the present value of lease payments over the term. As the Company’s leases do not provide readily determinable implicit interest rates, the Company utilized its incremental borrowing rate, which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. In transition to ASC 842, the Company utilized the remaining lease term of its leases in determining the appropriate incremental borrowing rates. The application of the new standard required netting of unamortized balance of lease incentives and deferred lease obligation to the right-of-use asset at the adoption date. The Company’s operating leases include rental escalation clauses that are factored into the determination of lease payments when appropriate. The Company does not separate lease and non-lease components of contracts. Refer to Note 3, Leases, to this Annual Report on Form 10-K for additional information. Share-Based Compensation In June 2018, the FASB issued ASU No. 2018-07, Stock-based Compensation: Improvements to Nonemployee Share-based Payment Accounting (Topic 718) ("ASU 2018-07"), which amends the existing accounting standards for share-based payments to nonemployees. This ASU aligns much of the guidance on measuring and classifying nonemployee awards with that of awards to employees. Under the new guidance, the measurement of nonemployee equity awards is fixed on the grant date. Entities will apply the ASU by recognizing a cumulative-effect adjustment, if any, to retained earnings as of the beginning of the annual period of adoption. The Company adopted ASU 2018-07 on January 1, 2020. The adoption of this standard did not have a material impact to this Annual Report on Form 10-K. To be adopted in future periods In December 2019, the FASB issued ASU No. 2019 -12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The new standard includes several provisions which simplify accounting for income taxes by removing certain exceptions to the general principles in Topic 740 and increasing consistency and clarity for the users of financial statements. This standard will be effective for the Company on January 1, 2021. Early adoption is permitted. The adoption of this guidance is not expected to have a material impact on the Company’s financial position and results of operations. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326)—Measurement of Credit Losses on Financial Instruments, which has been subsequently amended by ASU No. 2018-19, ASU No. 2019-04, ASU No. 2019-05, ASU No. 2019-10, ASU No. 2019-11 and ASU No. 2020-03 (“ASU 2016-13”). The provisions of ASU 2016-13 modify the impairment model to utilize an expected loss methodology in place of the currently used incurred loss methodology and require a consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 is effective for the Company on January 1, 2023, with early adoption permitted. The Company is currently evaluating the potential impact that this standard may have on its financial position and results of operations, as well as the timing of its adoption of this standard. On August 5, 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (“ASU-2020-06”), which simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. ASU 2020-06 eliminates the beneficial conversion and cash conversion accounting models in ASC 470-20 that require separate accounting for embedded conversion features from convertible instruments. As a result, after adopting the ASU’s guidance, entities will not separately present in equity an embedded conversion feature in such debt. Additionally, the guidance simplifies the evaluation of whether a contract in the issuer’s own equity can be classified in equity or an embedded feature qualifies for the derivative scope exception. Although the guidance is not effective until 2022, early adoption of ASU 2020-06 is permitted for all entities for fiscal years beginning after December 15, 2020. The Company is currently evaluating the impact of this new guidance on the Company’s consolidated financial statements and related disclosures.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted cash	The following reconciles cash, cash equivalents and restricted cash as of December 31, 2020 and 2019, as presented on the Company's consolidated statements of cash flows, to its related consolidated balance sheet accounts (in thousands): December 31, 2020 2019 Cash and cash equivalents: Cash $ 4,487 $ 1,634 Money market funds 64,370 76,199 Total cash and cash equivalents 68,857 77,833 Restricted cash 1,563 1,500 Cash, cash equivalents and restricted cash $ 70,420 $ 79,333
Reconciliation of Cash, Cash Equivalents and Restricted cash	The following reconciles cash, cash equivalents and restricted cash as of December 31, 2020 and 2019, as presented on the Company's consolidated statements of cash flows, to its related consolidated balance sheet accounts (in thousands): December 31, 2020 2019 Cash and cash equivalents: Cash $ 4,487 $ 1,634 Money market funds 64,370 76,199 Total cash and cash equivalents 68,857 77,833 Restricted cash 1,563 1,500 Cash, cash equivalents and restricted cash $ 70,420 $ 79,333
Summary of Estimated Useful Lives of Property and Equipment	A summary of the estimated useful lives is as follows: Classification Estimated Useful Life Computer hardware 3 - 5 years Computer software 3 years Furniture and fixtures 7 years Research and lab equipment (used/new) 3/5 years Leasehold improvements Lesser of asset life or Property and equipment consists of the following (in thousands): December 31, 2020 2019 Property and equipment: Lab equipment $ 8,831 $ 7,479 Leasehold improvements 2,157 2,014 Furniture and fixtures 822 750 Computers and software 230 204 Office equipment 3 9 Construction-in-process 1,078 1,594 Property and equipment 13,121 12,050 Less: accumulated depreciation (5,643) (3,709) Property and equipment, net $ 7,478 $ 8,341

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Schedule of Minimum Aggregate Future Lease Commitments	The minimum aggregate future lease commitments at December 31, 2020, are as follows (in thousands): Amount 2021 $ 2,727 2022 3,062 2023 3,154 2024 3,249 2025 2,491 Total lease payments 14,683 Less imputed interest (3,020) Total $ 11,663 Other information: Operating cash flows used for operating leases $ 3,334 Weighted-average remaining lease term (in years) 5 years Weighted-average discount rate 9.5 %
Schedule of Minimum Rental Commitments Under Non-cancelable Leases Under Prior Lease Accounting Guidance	Under the prior lease accounting guidance, minimum rental commitments under non-cancelable leases as of December 31, 2019 were as follows (in thousands): Amount 2021 $ 2,973 2022 3,062 2023 3,154 2024 3,249 2025 2,492 Total lease payments $ 14,930

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value	The following tables present information about the Company’s financial assets and liabilities that have been measured at fair value as of December 31, 2020 and 2019 (in thousands): Description December 31, 2020 Assets: Money market funds included within cash and cash equivalents $ 64,370 $ 64,370 $ — $ — Total $ 64,370 $ 64,370 $ — $ — Description December 31, 2019 Assets: Money market funds included within cash and cash equivalents $ 76,199 $ 76,199 $ — $ — Total $ 76,199 $ 76,199 $ — $ —

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	A summary of the estimated useful lives is as follows: Classification Estimated Useful Life Computer hardware 3 - 5 years Computer software 3 years Furniture and fixtures 7 years Research and lab equipment (used/new) 3/5 years Leasehold improvements Lesser of asset life or Property and equipment consists of the following (in thousands): December 31, 2020 2019 Property and equipment: Lab equipment $ 8,831 $ 7,479 Leasehold improvements 2,157 2,014 Furniture and fixtures 822 750 Computers and software 230 204 Office equipment 3 9 Construction-in-process 1,078 1,594 Property and equipment 13,121 12,050 Less: accumulated depreciation (5,643) (3,709) Property and equipment, net $ 7,478 $ 8,341

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): December 31, 2020 2019 Accrued external research and development expenses $ 9,394 $ 4,583 Accrued payroll and related expenses 5,620 3,149 Accrued professional fees 604 659 Accrued other 636 367 Total accrued expenses $ 16,254 $ 8,758

Loan and Security Agreement (Ta

Loan and Security Agreement (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Schedule of Minimum Aggregate Future Loan Payments	The Company has the following minimum aggregate future loan payments at December 31, 2020 (in thousands): Amount 2021 $ 2,631 2022 11,603 2023 13,387 2024 10,259 Total minimum payments $ 37,880 Less amounts representing interest and discount (7,832) Long-term debt $ 30,048

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Summary of Stock-based Compensation Expense	Stock-based compensation expense included in the Company’s statements of operations is as follows (in thousands): Year Ended December 31, 2020 2019 Research and development $ 4,487 $ 3,648 General and administrative 3,981 4,517 Total stock-based compensation expense $ 8,468 $ 8,165
Summary of Stock Option Activity	A summary of the Company’s stock option activity and related information is as follows: Shares Weighted Weighted Aggregate Options outstanding at December 31, 2019 5,691,474 $ 6.99 Granted 2,161,356 $ 6.06 Exercised (195,513) $ 1.94 Canceled (1,046,655) $ 8.80 Options outstanding at December 31, 2020 6,610,662 $ 6.55 7.53 $ 38,815 Exercisable at December 31, 2020 3,671,175 $ 5.52 6.77 $ 25,332 Vested and expected to vest as of December 31, 2020 6,610,662 $ 6.55 7.53 $ 38,815 (1) The aggregate intrinsic value of 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 exercise prices lower than the fair value of the common stock as of the end of the period.
Schedule of Stock Option Valuation Assumptions	When utilizing the Black-Scholes option-pricing model to determine the grant date fair value of stock options granted to employees or non-employees, the Company used the following weighted average, or ranges of, assumptions: Employee option grants Year Ended December 31, 2020 2019 Risk-free interest rate 1.11 % 2.28 % Expected life (in years) 6.05 6.02 Volatility 79.6 % 76.2 % Expected dividend rate 0.00 % 0.00 % Non-employee option grants Year Ended December 31, 2020 2019 Risk-free interest rate 0.38 % 1.98 % Expected life (in years) 5.21 7.63 Volatility 78.9 % 76.0 % Expected dividend rate 0.00 % 0.00 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	The Company’s loss before income tax for the periods presented was generated in the United States with a small profit generated by the Company's subsidiary in the United Kingdom. December 31, 2020 2019 U.S. federal tax statutory rate 21.0 % 21.0 % State taxes, net of federal benefit 6.8 % 7.0 % Non-deductible stock compensation (1.0) % (0.6) % Other non-deductible expenses (0.4) % (0.4) % Credits 1.8 % 1.6 % Change in valuation allowance (28.6) % (29.1) % Other — % 0.3 % Total (0.4) % (0.2) %
Schedule of Deferred Tax Assets	December 31, 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 36,256 $ 25,895 Research and development credits 7,092 4,856 Capitalized research and development, patent and start-up costs 34,452 22,101 Accrued expenses 1,370 1,006 Stock based compensation 3,443 2,335 Operating lease liability 3,186 — Right of use asset - operating lease (2,939) — Depreciation (208) (295) Deferred tax assets before valuation allowance 82,652 55,898 Valuation allowance (82,652) (55,898) Net deferred tax assets $ — $ —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted-Average Shares Outstanding	The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive: Year Ended December 31, 2020 2019 Unvested common stock from early exercise of options 18,386 61,653 Stock options to purchase common stock 6,610,662 5,691,474 RSUs 284,000 — Common stock from the ESPP 24,508 — Total 6,937,556 5,753,127

Organization and Basis of Pre_2

Organization and Basis of Presentation - Narrative (Details) - USD ($)	Feb. 02, 2021	Jan. 28, 2021	Jul. 14, 2020	Jun. 03, 2019	Jan. 31, 2021	Jun. 30, 2020	Mar. 09, 2021	Dec. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from the issuance of long-term debt								$ 10,000,000	$ 19,481,000
Net losses								93,666,000	85,472,000
Accumulated deficit								292,519,000	198,853,000
Cash and cash equivalents								$ 68,857,000	$ 77,833,000
Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from transaction							$ 82,200,000
Security and Loan Agreement, Tranche Two | Line of Credit | 2019 Credit Facility
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from the issuance of long-term debt			$ 10,000,000
Registration Statement
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Registration of equity instruments, aggregate authorized amount				$ 200,000,000
Sale of stock, term				3 years
At-the-market
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Registration of equity instruments, aggregate authorized amount				$ 50,000,000
Number of shares issued (in shares)								1,232,131
Gross proceeds from transaction								$ 6,800,000
Net proceeds from transaction								$ 6,600,000
At-the-market | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of shares issued (in shares)					139,734
Gross proceeds from transaction					$ 1,800,000
Net proceeds from transaction					$ 1,700,000
At-the-market | Minimum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Offering price per share (in dollars per share)								$ 4.25
At-the-market | Minimum | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Offering price per share (in dollars per share)					$ 12.54
At-the-market | Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Offering price per share (in dollars per share)								$ 11.15
At-the-market | Maximum | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Offering price per share (in dollars per share)					$ 13.17
Public offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of shares issued (in shares)						13,800,000
Offering price per share (in dollars per share)						$ 3.75
Gross proceeds from transaction						$ 51,800,000
Net proceeds from transaction						$ 48,400,000
Public offering | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of shares issued (in shares)	5,175,000
Offering price per share (in dollars per share)	$ 15
Gross proceeds from transaction	$ 77,600,000
Net proceeds from transaction	$ 73,000,000
Over-allotment option
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of shares issued (in shares)						1,800,000
Over-allotment option | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of shares issued (in shares)	675,000
Private placement | Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of shares issued (in shares)		7,500,000
Offering price per share (in dollars per share)		$ 15

Significant Accounting Polici_4

Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2020USD ($)segment	Dec. 31, 2019USD ($)	Jan. 01, 2020USD ($)
Accounting Policies [Abstract]
Comprehensive loss	$ 93,666,000	$ 85,454,000
Current restricted cash	300,000	0
Noncurrent restricted cash	$ 1,300,000	$ 1,500,000
Number of operating segments | segment	1
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset	$ 10,757,000
Operating lease liability	$ 11,663,000
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset			$ 12,700,000
Operating lease liability			$ 13,900,000

Significant Accounting Polici_5

Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash and cash equivalents:
Cash	$ 4,487	$ 1,634
Money market funds	64,370	76,199
Total cash and cash equivalents	68,857	77,833
Restricted cash	1,563	1,500
Cash, cash equivalents and restricted cash	$ 70,420	$ 79,333	$ 94,351

Significant Accounting Polici_6

Significant Accounting Policies - Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2020
Computer hardware | Minimum
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	3 years
Computer hardware | Maximum
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	5 years
Computer software
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	3 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	7 years
Research and lab equipment, used
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	3 years
Research and lab equipment, new
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	5 years

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Jan. 31, 2018ft²
Leases [Abstract]
Area of leased office and research development space (in square feet) | ft²			40,765
Minimum rental payments received	$ 0.2
Rent expense	2.9
Rent expense		$ 2.9
Sublease rental income	$ 0.3
Sublease rental income		$ 0.5

Leases - Schedule of Minimum Ag

Leases - Schedule of Minimum Aggregate Future Lease Commitments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Commitments and Contingencies Disclosure [Abstract]
2021	$ 2,727
2022	3,062
2023	3,154
2024	3,249
2025	2,491
Total lease payments	14,683
Less imputed interest	(3,020)
Total	11,663
Other information:
Operating cash flows used for operating leases	$ 3,334
Weighted-average remaining lease term (in years)	5 years
Weighted-average discount rate	9.50%

Leases - Schedule of Minimum Re

Leases - Schedule of Minimum Rental Commitments Under Non-cancelable Leases Under Prior Lease Accounting Guidance (Details) $ in Thousands	Dec. 31, 2019USD ($)
Leases [Abstract]
2021	$ 2,973
2022	3,062
2023	3,154
2024	3,249
2025	2,492
Total lease payments	$ 14,930

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Total	$ 64,370	$ 76,199
Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	64,370	76,199
(Level 1)
Assets:
Total	64,370	76,199
(Level 1) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	64,370	76,199
(Level 2)
Assets:
Total	0	0
(Level 2) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	0	0
(Level 3)
Assets:
Total	0	0
(Level 3) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	$ 0	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property and equipment:
Property and equipment	$ 13,121	$ 12,050
Less: accumulated depreciation	(5,643)	(3,709)
Property and equipment, net	7,478	8,341
Lab equipment
Property and equipment:
Property and equipment	8,831	7,479
Leasehold improvements
Property and equipment:
Property and equipment	2,157	2,014
Furniture and fixtures
Property and equipment:
Property and equipment	822	750
Computers and software
Property and equipment:
Property and equipment	230	204
Office equipment
Property and equipment:
Property and equipment	3	9
Construction-in-process
Property and equipment:
Property and equipment	$ 1,078	$ 1,594

Property and Equipment, Net - N

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 2,026	$ 1,764

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 9,394	$ 4,583
Accrued payroll and related expenses	5,620	3,149
Accrued professional fees	604	659
Accrued other	636	367
Total accrued expenses	$ 16,254	$ 8,758

Loan and Security Agreement - N

Loan and Security Agreement - Narrative (Details)	Jul. 14, 2020USD ($)	Jul. 19, 2019USD ($)tranche	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Jan. 15, 2021USD ($)	Dec. 31, 2016USD ($)
Line of Credit Facility [Line Items]
Net proceeds from the issuance of long-term debt			$ 10,000,000	$ 19,481,000
2016 Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity						$ 15,000,000
Interest expense				500,000
2016 Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 15,000,000
2019 Credit Facility
Line of Credit Facility [Line Items]
Interest expense			$ 2,600,000	$ 800,000
2019 Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 45,000,000
Number of tranches | tranche		3
Interest rate		8.65%
Final payment under lease agreement as a percentage of loans borrowed		4.30%
Fees incurred to establish facility		$ 400,000
Prepayment fee percentage		2.00%
Anniversary of funding date for determining prepayment fee percentage		18 months
Prepayment fee percentage if the prepayment occurs after the 18-month anniversary of the loan funding date		1.00%
Debt default interest rate per annum		5.00%
2019 Credit Facility | Line of Credit | Prime Plus
Line of Credit Facility [Line Items]
Basis spread on interest rate		3.15%
2019 Credit Facility | Security and Loan Agreement, Tranche One | Line of Credit
Line of Credit Facility [Line Items]
Net proceeds from the issuance of long-term debt		$ 20,000,000
2019 Credit Facility | Security and Loan Agreement, Tranche Two | Line of Credit
Line of Credit Facility [Line Items]
Net proceeds from the issuance of long-term debt	$ 10,000,000
2019 Credit Facility | Security and Loan Agreement, Tranche Three | Line of Credit | Subsequent Event
Line of Credit Facility [Line Items]
Amount expired					$ 15,000,000

Loan and Security Agreement - S

Loan and Security Agreement - Schedule of Minimum Aggregate Future Loan Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Long-term debt	$ 30,048	$ 19,634
Line of Credit
Line of Credit Facility [Line Items]
2021	2,631
2022	11,603
2023	13,387
2024	10,259
Total minimum payments	37,880
Less amounts representing interest and discount	(7,832)
Long-term debt	$ 30,048

In-License Agreements - Narrati

In-License Agreements - Narrative (Details) - USD ($)	Aug. 06, 2017	Mar. 10, 2016	Dec. 31, 2020	Dec. 31, 2020
Mayo Clinic
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-refundable upfront fee	$ 200,000
Milestone payments upon achievement of certain development, regulatory, and commercial events			$ 200,000
Amount due under license agreement			0	$ 0
Mayo Clinic | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments upon achievement of certain development, regulatory, and commercial events	$ 56,000,000
University of Chicago
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments upon achievement of certain development, regulatory, and commercial events		$ 60,900,000
Amount due under license agreement			$ 0	0
Milestone payments for development and commercialization of licensed products				$ 400,000
University of Chicago | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-refundable upfront fee		$ 500,000

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) € in Millions, $ in Millions	1 Months Ended	18 Months Ended
	Jul. 31, 2019EUR (€)	Dec. 31, 2020EUR (€)	Dec. 31, 2020USD ($)
Biose Industrie
Commitment And Contingencies [Line Items]
Purchase obligation | $			$ 0.7
Collaborative Arrangement | Sacco
Commitment And Contingencies [Line Items]
Term of collaboration arrangement	5 years
Period of inactive manufacturing services causing termination under collaborative agreement	6 months
Aggregate amount due under collaborative arrangement	€ 3
Annual amount due under collaborative arrangement	€ 0.6
Fee Incurred Under Collaborative Arrangement		€ 1.2

Stockholders_ Equity (Details)

Stockholders’ Equity (Details) - USD ($)	Jun. 03, 2019	Jun. 30, 2020	Dec. 31, 2020
Registration Statement
Subsidiary, Sale of Stock [Line Items]
Registration of equity instruments, aggregate authorized amount	$ 200,000,000
Sale of stock, term	3 years
At-the-market
Subsidiary, Sale of Stock [Line Items]
Registration of equity instruments, aggregate authorized amount	$ 50,000,000
Number of shares issued (in shares)			1,232,131
Gross proceeds from transaction			$ 6,800,000
Net proceeds from transaction			$ 6,600,000
At-the-market | Minimum
Subsidiary, Sale of Stock [Line Items]
Offering price per share (in dollars per share)			$ 4.25
At-the-market | Maximum
Subsidiary, Sale of Stock [Line Items]
Offering price per share (in dollars per share)			$ 11.15
Public offering
Subsidiary, Sale of Stock [Line Items]
Number of shares issued (in shares)		13,800,000
Offering price per share (in dollars per share)		$ 3.75
Gross proceeds from transaction		$ 51,800,000
Net proceeds from transaction		$ 48,400,000

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2021	Nov. 04, 2020	Jan. 01, 2020	Jan. 01, 2019	May 08, 2018	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2015	Dec. 31, 2020	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options granted (in shares)							2,161,356
Number of options canceled (in shares)							1,046,655
Number of options exercised (in shares)							195,513
Unvested stock options outstanding (in shares)						2,957,873	2,957,873			2,957,873	2,957,873
Weighted average fair value of options granted (in dollars per share)							$ 4.14	$ 7.46
Aggregate intrinsic value of options exercised							$ 900	$ 1,800
Compensation expense							$ 8,468	$ 8,165
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield							0.00%
Unrecognized stock based compensation expense, stock options						$ 14,400	$ 14,400			$ 14,400	$ 14,400
Compensation cost not yet recognized, period for recognition							2 years 1 month 13 days
Stock Option | Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield							0.00%	0.00%
Stock Option | Non-employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield							0.00%	0.00%
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of RSUs granted (in shares)		284,000
Weighted average grant date fair value of RSUs (in dollars per share)		$ 4.41
RSUs vested (in shares)						0
RSUs | First anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting percentage		25.00%
RSUs | Second anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting percentage		25.00%
RSUs | Third anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting percentage		50.00%
2018 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of outstanding shares					4.00%
Additional shares authorized for issuance (in shares)			1,286,824	1,273,031
Number of options granted (in shares)										4,376,182
Number of options canceled (in shares)										875,155
Number of options exercised (in shares)										4,640
Number of common stock available for future grant (in shares)						951,621	951,621			951,621	951,621
2018 Stock Incentive Plan | Stock Option | Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2018 Stock Incentive Plan | RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of RSUs granted (in shares)										284,000
2018 Stock Incentive Plan | Minimum | Stock Option | Non-employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
2018 Stock Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for issuance (in shares)					1,344,692
2018 Stock Incentive Plan | Maximum | Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, maximum expiration period					10 years
2018 Stock Incentive Plan | Maximum | Stock Option | Non-employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2018 Stock Incentive Plan | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized for issuance (in shares)	1,898,805
2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options canceled (in shares)							832,101
Number of common stock available for future grant (in shares)									5,417,044
Number of shares, options and other equity awards granted (in shares)											5,758,518
Number of shares, options and other equity awards exercised (in shares)											1,376,141
Number of shares, options and other equity awards canceled (in shares)											1,268,110
Number of shares, options and other equity awards repurchased (in shares)											18,468
Number of shares no longer authorized (in shares)									113,006
2015 Stock Incentive Plan | Stock Option | Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									4 years
2015 Stock Incentive Plan | Minimum | Stock Option | Non-employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									1 year
2015 Stock Incentive Plan | Maximum | Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, maximum expiration period									10 years
2015 Stock Incentive Plan | Maximum | Stock Option | Non-employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									4 years
2018 Employee Stock Purchase Plan | Employee stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of outstanding shares					1.00%
Shares reserved for issuance					336,356
Compensation expense							$ 100
Number of shares issued for purchase (in shares)							28,603
2018 Employee Stock Purchase Plan | Subsequent Event | Employee stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized for issuance (in shares)	474,701

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 8,468	$ 8,165
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	4,487	3,648
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 3,981	$ 4,517

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020
Shares
Options outstanding, beginning balance (in shares)	5,691,474
Granted (in shares)	2,161,356
Exercised (in shares)	(195,513)
Canceled (in shares)	(1,046,655)
Options outstanding, ending balance (in shares)	6,610,662
Exercisable at end of period (in shares)	3,671,175
Vested and expected to vest at end of period (in shares)	6,610,662
Weighted Average - Exercise Price
Options outstanding, beginning balance (in dollars per share)	$ 6.99
Granted (in dollars per share)	6.06
Exercised (in dollars per share)	1.94
Canceled (in dollars per share)	8.80
Options outstanding, ending balance (in dollars per share)	6.55
Exercisable at end of period (in dollars per share)	5.52
Vested and expected to vest at end of period (in dollars per share)	$ 6.55
Weighted Average - Remaining Contractual Life
Options, outstanding, weighted average remaining contractual life	7 years 6 months 10 days
Options, exercisable, weighted average remaining contractual life	6 years 9 months 7 days
Options, vested and expected to vest, weighted average remaining contractual life	7 years 6 months 10 days
Options, outstanding, aggregate intrinsic value	$ 38,815
Options, exercisable, aggregate intrinsic value	25,332
Options, vested and expected to vest, aggregate intrinsic value	$ 38,815

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock Option Valuation Assumptions (Details) - Stock Option	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend rate	0.00%
Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.11%	2.28%
Expected life (in years)	6 years 18 days	6 years 7 days
Volatility	79.60%	76.20%
Expected dividend rate	0.00%	0.00%
Non-employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.38%	1.98%
Expected life (in years)	5 years 2 months 15 days	7 years 7 months 17 days
Volatility	78.90%	76.00%
Expected dividend rate	0.00%	0.00%

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Line Items]
Income tax provision	$ 409,000	$ 190,000
Percentage of income limitation on carryback provisions	80.00%
Increase in valuation allowance	$ 26,800,000
Unrecognized tax benefits	0	$ 0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	133,700,000
Net operating loss carryforwards subject to expiration	49,900,000
Net operating loss carried forward indefinitely	83,800,000
Research credit carryforwards	5,000,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	129,400,000
Research credit carryforwards	$ 2,600,000

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
U.S. federal tax statutory rate	21.00%	21.00%
State taxes, net of federal benefit	6.80%	7.00%
Non-deductible stock compensation	(1.00%)	(0.60%)
Other non-deductible expenses	(0.40%)	(0.40%)
Credits	1.80%	1.60%
Change in valuation allowance	(28.60%)	(29.10%)
Other	0.00%	0.30%
Effective income tax rate reconciliation	(0.40%)	(0.20%)

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 36,256	$ 25,895
Research and development credits	7,092	4,856
Capitalized research and development, patent and start-up costs	34,452	22,101
Accrued expenses	1,370	1,006
Stock based compensation	3,443	2,335
Operating lease liability	3,186	0
Right of use asset - operating lease	(2,939)	0
Depreciation	(208)	(295)
Deferred tax assets before valuation allowance	82,652	55,898
Valuation allowance	(82,652)	(55,898)
Net deferred tax assets	$ 0	$ 0

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted-Average Shares Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from the computation of diluted weighted-average shares outstanding	6,937,556	5,753,127
Unvested common stock from early exercise of options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from the computation of diluted weighted-average shares outstanding	18,386	61,653
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from the computation of diluted weighted-average shares outstanding	6,610,662	5,691,474
RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from the computation of diluted weighted-average shares outstanding	284,000	0
Common stock from the ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from the computation of diluted weighted-average shares outstanding	24,508	0

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) $ in Millions	Oct. 11, 2020installmentshares	Sep. 16, 2019USD ($)installmentshares	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)
Related Party Transaction [Line Items]
Sublease rental income			$ 0.3
Number of options granted (in shares) | shares			2,161,356
Weatherden Ltd
Related Party Transaction [Line Items]
Payment to related party			$ 0.6	$ 1
Amounts due to related party				$ 0.2
VL46
Related Party Transaction [Line Items]
Rent due, sublease agreement			0.9
Sublease rental income			$ 0.3
Consulting Agreement | Mr. Epstein
Related Party Transaction [Line Items]
Termination notice period of consulting arrangement		30 days
Termination notice period by non-breaching party in event of a breach		24 hours
Number of options granted (in shares) | shares	44,743	75,000
Option to purchase shares, vesting period, number of equal monthly installments | installment		36
Aggregate grant date fair value		$ 0.2
Annual equity award, vesting period, number of equal monthly installments | installment	9	12
Aggregate annual cash consulting fee		$ 0.3

Defined Contribution Plan (Deta

Defined Contribution Plan (Details) - U.S. Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employee deferral percentage	85.00%
Defined contribution plan, matching contribution expense	$ 0.3	$ 0.2

Subsequent Event (Details)

Subsequent Event (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 02, 2021	Jan. 28, 2021	Jan. 31, 2021	Jun. 30, 2020	Mar. 09, 2021	Dec. 31, 2020
Subsequent Event
Subsequent Event [Line Items]
Net proceeds from transaction					$ 82.2
At-the-market
Subsequent Event [Line Items]
Number of shares issued (in shares)						1,232,131
Gross proceeds from transaction						$ 6.8
Net proceeds from transaction						$ 6.6
At-the-market | Minimum
Subsequent Event [Line Items]
Offering price per share (in dollars per share)						$ 4.25
At-the-market | Maximum
Subsequent Event [Line Items]
Offering price per share (in dollars per share)						$ 11.15
At-the-market | Subsequent Event
Subsequent Event [Line Items]
Number of shares issued (in shares)			139,734
Gross proceeds from transaction			$ 1.8
Net proceeds from transaction			$ 1.7
At-the-market | Subsequent Event | Minimum
Subsequent Event [Line Items]
Offering price per share (in dollars per share)			$ 12.54
At-the-market | Subsequent Event | Maximum
Subsequent Event [Line Items]
Offering price per share (in dollars per share)			$ 13.17
Public offering
Subsequent Event [Line Items]
Number of shares issued (in shares)				13,800,000
Offering price per share (in dollars per share)				$ 3.75
Gross proceeds from transaction				$ 51.8
Net proceeds from transaction				$ 48.4
Public offering | Subsequent Event
Subsequent Event [Line Items]
Number of shares issued (in shares)	5,175,000
Offering price per share (in dollars per share)	$ 15
Gross proceeds from transaction	$ 77.6
Net proceeds from transaction	$ 73
Over-allotment option
Subsequent Event [Line Items]
Number of shares issued (in shares)				1,800,000
Over-allotment option | Subsequent Event
Subsequent Event [Line Items]
Number of shares issued (in shares)	675,000
Private placement | Subsequent Event
Subsequent Event [Line Items]
Number of shares issued (in shares)		7,500,000
Offering price per share (in dollars per share)		$ 15