Evelo Biosciences (EVLO) 10-K 15 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 08, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	EVLO
Entity Registrant Name	Evelo Biosciences, Inc.
Entity Central Index Key	1,694,665
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Smaller Reporting Company	true
Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		31,869,740
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well Known Seasoned Issuer	No
Public Float			$ 158

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 93,101	$ 38,246
Short-term investments	54,818	0
Prepaid expenses and other current assets	3,703	531
Total current assets	151,622	38,777
Property and equipment, net	6,925	3,496
Other assets	1,320	1,515
Total assets	159,867	43,788
Current liabilities:
Accounts payable	1,519	1,411
Accrued expenses	4,965	2,199
Other current liabilities	2,751	229
Total current liabilities	9,235	3,839
Noncurrent liabilities:
Long-term debt, net of current portion	12,305	9,966
Deferred rent, net of current portion	1,071	478
Other noncurrent liabilities	307	526
Total liabilities	22,918	14,809
Commitments and contingencies
Convertible preferred stock:
Convertible preferred stock, $0.001 par value; none and 66,311,563 shares authorized at December 31, 2018 and 2017, respectively; none and 65,833,096 shares issued and outstanding at December 31, 2018 and 2017, respectively; aggregate liquidation preference of $0 and $89,975 at December 31, 2018 and 2017, respectively	0	83,702
Stockholder’s equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 and no shares authorized at December 31, 2018 and 2017, respectively; no shares issued and outstanding at December 31, 2018 and 2017, respectively	0	0
Common stock, $0.001 par value; 200,000,000 and 23,780,338 shares authorized at December 31, 2018 and 2017, respectively; 31,951,540 and 4,138,483 shares issued and 31,825,769 and 3,880,607 shares outstanding at December 31, 2018 and 2017, respectively	32	4
Additional paid-in capital	250,316	1,684
Accumulated other comprehensive loss	(18)	0
Accumulated deficit	(113,381)	(56,411)
Total stockholders’ equity (deficit)	136,949	(54,723)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 159,867	$ 43,788

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized (in shares)	0	66,311,563
Convertible preferred stock, shares issued (in shares)	0	65,833,096
Convertible preferred stock, shares outstanding (in shares)	0	65,833,096
Convertible preferred stock, liquidation preference	$ 0	$ 89,975
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, authorized (in shares)	10,000,000	0
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	23,780,338
Common stock, shares issued (in shares)	31,951,540	4,138,483
Common stock, shares outstanding (in shares)	31,825,769	3,880,607

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 39,885	$ 19,957	$ 9,134
General and administrative	18,218	7,574	3,891
Total operating expenses	58,103	27,531	13,025
Loss from operations	(58,103)	(27,531)	(13,025)
Other income (expense):
Interest income (expense), net	1,563	(215)	(287)
Other expenses	(406)	(301)	(20)
Other income (expense), net	1,157	(516)	(307)
Net loss	(56,946)	(28,047)	(13,332)
Convertible preferred stock dividends	(3,937)	(6,085)	(1,645)
Net loss attributable to common stockholders	$ (60,883)	$ (34,132)	$ (14,977)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (2.78)	$ (9.10)	$ (5.28)
Weighted-average number of common shares outstanding, basic and diluted (in shares)	21,871,029	3,750,790	2,834,733
Comprehensive loss:
Net loss	$ (56,946)	$ (28,047)	$ (13,332)
Other comprehensive loss:
Unrealized loss on investments, net of tax of $0	(18)	0	0
Comprehensive loss	$ (56,964)	$ (28,047)	$ (13,332)

Consolidated Statements of Op_2

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Tax portion of unrealized loss on investments	$ 0	$ 0	$ 0

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Convertible Preferred Stock, Series A-1 and A-3	Convertible Preferred Stock, Series A and A-2	Convertible Preferred Stock, Series B	Convertible Preferred Stock, Series B and C	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2015		12,536,945
Beginning balance at Dec. 31, 2015		$ 7,773
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series A-1 and A-3 Preferred Stocks and Common Stock as part of the acquisition of Epiva (in shares)			18,851,705
Issuance of Series A-1 and A-3 Preferred Stocks and Common Stock as part of the acquisition of Epiva			$ 16,950
Issuance of preferred stock for cash, net of issuance costs (in shares)				6,666,668
Issuance of preferred stock for cash, net of issuance costs				$ 7,495
Accretion of preferred stock to redemption value		$ 1,645
Ending balance (in shares) at Dec. 31, 2016		38,055,318
Ending balance at Dec. 31, 2016		$ 33,863
Beginning balance (in shares) at Dec. 31, 2015							1,961,265
Beginning balance at Dec. 31, 2015	$ (4,471)						$ 2	$ 0	$ 0	$ (4,473)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Series A-1 and A-3 Common Stock as part of the acquisition of Epiva (in shares)							1,389,939
Issuance of Series A-1 and A-3 Common Stock as part of the acquisition of Epiva	(9,407)						$ 2			(9,409)
Vesting of restricted common stock (in shares)							216,146
Vesting of restricted common stock	$ 67							67
Exercise of stock options (in shares)	640,268						51,193
Exercise of stock options and warrants	$ 32							32
Accretion of preferred stock to redemption value	(1,645)							(518)		(1,127)
Stock-based compensation expense	419							419
Unrealized loss on investments	0
Net loss	(13,332)									(13,332)
Ending balance (in shares) at Dec. 31, 2016							3,618,543
Ending balance at Dec. 31, 2016	$ (28,337)						$ 4	0	0	(28,341)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of preferred stock for cash, net of issuance costs (in shares)					27,777,778
Issuance of preferred stock for cash, net of issuance costs					$ 49,807
Accretion of preferred stock to redemption value		$ 32
Ending balance (in shares) at Dec. 31, 2017	65,833,096	65,833,096
Ending balance at Dec. 31, 2017	$ 83,702	$ 83,702
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)							154,920
Vesting of restricted common stock	57							57
Exercise of stock options (in shares)							106,654
Exercise of stock options and warrants	79							79
Other issuances of common stock (in shares)							490
Other issuances of common stock	15							15
Accretion of preferred stock to redemption value	(32)							(9)		(23)
Stock-based compensation expense	1,542							1,542
Unrealized loss on investments	0
Net loss	(28,047)									(28,047)
Ending balance (in shares) at Dec. 31, 2017							3,880,607
Ending balance at Dec. 31, 2017	(54,723)						$ 4	1,684	0	(56,411)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Retroactive adjustment to beginning accumulated deficit and additional paid-in capital resulting from adoption of ASU 2016-09 | Accounting Standards Update 2016-09								24		(24)
Issuance of preferred stock for cash, net of issuance costs (in shares)						25,482,199
Issuance of preferred stock for cash, net of issuance costs						$ 82,076
Conversion of preferred stock into common stock (in shares)		(91,315,295)					22,386,677
Conversion of preferred stock into common stock	$ 165,778	$ (165,778)					$ 22	165,756
Ending balance (in shares) at Dec. 31, 2018	0	0
Ending balance at Dec. 31, 2018	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from Initial Public Offering, net of underwriting costs and commissions (in shares)							5,312,500
Proceeds from Initial Public Offering, net of underwriting costs and commissions	75,814						$ 5	75,809
Reclassification of warrant liability	819							819
Vesting of restricted common stock (in shares)							113,641
Vesting of restricted common stock	$ 36							36
Exercise of stock options (in shares)	84,801						132,344
Exercise of stock options and warrants	$ 130						$ 1	129
Accretion of preferred stock to redemption value	0
Stock-based compensation expense	6,059							6,059
Unrealized loss on investments	(18)								(18)
Net loss	(56,946)									(56,946)
Ending balance (in shares) at Dec. 31, 2018							31,825,769
Ending balance at Dec. 31, 2018	$ 136,949						$ 32	$ 250,316	$ (18)	$ (113,381)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (56,946)	$ (28,047)	$ (13,332)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,059	1,542	419
Depreciation expense	1,935	834	495
Change in fair value of warrant and debt derivative liability	406	301	20
Non-cash interest expense	103	35	28
Changes in assets and liabilities excluding effect of assets and liabilities assumed in 2016 acquisition of Epiva (Note 7):
Prepaid expenses and other current assets	(3,052)	(347)	(3)
Accounts payable	(221)	774	(43)
Accrued expenses and other current liabilities	3,594	1,733	273
Other liabilities	843	(90)	(171)
Net cash used in operating activities	(47,279)	(23,265)	(12,314)
Investing activities
Purchase of investments	(136,087)	0	0
Proceeds from sales and maturities of investments	81,250	0	0
Cash acquired in the 2016 acquisition of Epiva	0	0	10,486
Purchases of property and equipment	(5,462)	(1,742)	(1,250)
Proceeds from the sale of fixed assets	171	0	27
Net cash used in investing activities	(60,128)	(1,742)	9,263
Financing activities
Net proceeds from the issuance of common stock upon completion of initial public offering	75,829	0	0
Deferred issuance costs	0	(15)	0
Net proceeds from the issuance of convertible preferred stock	81,336	48,903	7,495
Net proceeds from the issuance of long-term debt	4,975	0	11,000
Settlement of derivative liability	(250)	0	0
Proceeds from the exercise of stock options, restricted common stock and warrants	122	79	247
Repayment of long-term debt	0	0	(4,000)
Change in stockholders' payable	0	0	1,000
Net cash provided by financing activities	162,012	48,967	15,742
Net increase in cash, cash equivalents and restricted cash	54,605	23,960	12,691
Cash, cash equivalents and restricted cash – beginning of year	39,746	15,786	3,095
Cash, cash equivalents and restricted cash – end of year	94,351	39,746	15,786
Supplemental disclosure of cash flow information
Cash paid for interest	742	437	204
Noncash investing and financing activities
Conversion of convertible preferred stock into common stock upon closing of initial public offering	165,778	0	0
Conversion of convertible preferred stock warrants into common stock warrants	819	0	0
Property and equipment additions in accounts payable and accrued expenses	348	84	0
Issuance of debt derivative liability in connection with long-term debt facility	150	0	0
Issuance of warrants in connection with long-term debt facility	89	0	76
Accretion of convertible preferred stock to redemption value	0	32	1,645
Long-term debt assumed in acquisition of Epiva, net of discount	0	0	2,923
Net non-cash assets acquired in acquisition of Epiva	$ 0	$ 0	$ 57

Organization and Basis of Prese

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
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 focuses on the development of monoclonal microbials, which are designed to act on the gut-body network for the treatment of many diseases, beginning with inflammatory diseases and cancer. The Company is headquartered in Cambridge, Massachusetts. The Company is devoting 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. On April 27, 2018, the Company filed an amended and restated certificate of incorporation with the Secretary of State of the State of Delaware, to effect a 1-for-4.079 reverse stock split of the Company’s common stock. All share and per share data shown in the accompanying consolidated financial statements and related notes have been retroactively revised to reflect the reverse stock split. On May 11, 2018, the Company completed an initial public offering (the “IPO”) of 5,312,500 shares of its common stock for aggregate gross proceeds of $85.0 million . The Company received $75.8 million in net proceeds after deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company. Upon closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 22,386,677 shares of common stock at the applicable conversion ratio then in effect. The Company has incurred operating losses since inception and expects such losses and negative operating cash flows to continue for the foreseeable future. Through December 31, 2018 , the Company has raised gross proceeds of approximately $257.3 million from the sale of common stock, the sale of convertible preferred stock, and from the issuance of debt. At December 31, 2018 , the Company had cash, cash equivalents and short-term investments of $147.9 million . The Company recorded net losses of $56.9 million , $28.0 million and $13.3 million for the years ended December 31, 2018 , 2017 and 2016, respectively. As of December 31, 2018 , the Company had an accumulated deficit of $113.4 million . The transition to profitability is dependent upon the successful development, approval, and commercialization of its products and product candidates and the achievement of a level of revenues adequate to support its cost structure. Based on the Company’s current operating plan, the Company believes that its cash, cash equivalents and short-term investments at December 31, 2018 will be sufficient to fund operations and capital expenditures for at least the twelve months following the filing of this Annual Report on Form 10-K. Management’s belief with respect to its ability to fund operations is based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional funding sooner than would otherwise be expected. There can be no assurance that the Company will be able to obtain additional funding on acceptable terms, if 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, 2018
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 condensed 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 subsidiary, Evelo Biosciences Security Corporation. All intercompany transactions and balances of the subsidiary 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 this Annual Report on Form 10-K. Emerging Growth Company Status Evelo is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act of 2012, or 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 this offering 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; or has more than $700.0 million in market value of its stock held by non-affiliates (and it has been a public company for at least 12 months and has filed one annual report on Form 10-K); or it issues 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, cash equivalents and short-term investments. The Company places its cash, cash equivalents and short-term investments in primarily two custodian accounts at accredited financial institutions. The Company’s available-for-sale investments primarily consist of U.S. Treasury securities. The Company has not experienced any realized losses on any of its accounts and management believes such funds are subject to minimal credit risk. Such deposits have and will continue to exceed federally insured limits. As of December 31, 2018 and 2017 , 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 (deficit) that are excluded from net loss. The Company's only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments. Comprehensive loss totaled $57.0 million for the year ended December 31, 2018 and was not significantly different than net loss. For the years ended December 31, 2017 and 2016 comprehensive loss was equal to 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. Cash equivalents are stated at cost, which approximates market value. The Company’s restricted cash consists of restricted cash in connection with building leases for the Company’s office and laboratory premises. Restricted cash as of December 31, 2018 and 2017 was $1.3 million and $1.5 million , respectively, and is classified within other assets on the accompanying consolidated balance sheet. The following reconciles cash, cash equivalents and restricted cash as of December 31, 2018 and 2017 , as presented on our statements of cash flows to their related balance sheet accounts (in thousands): December 31, 2018 2017 Cash and cash equivalents: Cash $ 1,300 $ 13,204 Money Market Funds 91,801 25,042 Total cash and cash equivalents 93,101 38,246 Restricted cash 1,250 1,500 Cash, cash equivalents and restricted cash $ 94,351 $ 39,746 Investments The Company accounts for and classifies its investments as either “available-for-sale,” “trading,” or “held-to-maturity,” in accordance with the accounting guidance related to the accounting and classification of certain investments in debt and equity securities. The determination of the appropriate classification is based primarily on management’s intent to sell the investment at the time of purchase. As of December 31, 2018 , all of the Company's investments were classified as available‑for‑sale securities. Available‑for‑sale securities are those securities which the Company views as available for use in current operations, if needed. The Company generally classifies its available‑for‑sale securities as short‑term investments, even though the stated maturity date may be one year or more beyond the current balance sheet date. Available‑for‑sale investments are stated at fair value with their unrealized gains and losses included in accumulated other comprehensive loss within stockholders’ (deficit) equity, until such gains and losses are realized in other income (expense) within the consolidated statements of operations and comprehensive loss or until an unrealized loss is considered other‑than‑temporary. The Company recognizes other‑than‑temporary impairments of its investments in debt securities when there is a decline in fair value below the amortized cost basis and if (a) it has the intent to sell the security or (b) it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. If either of these conditions is met, the Company recognizes the difference between the amortized cost of the security and its fair value at the impairment measurement date in the consolidated statements of operations and comprehensive loss. If neither of these conditions is met, the Company must perform additional analyses to evaluate whether the unrealized loss is associated with the creditworthiness of the issuer of the security rather than other factors, such as interest rates or market factors. If the Company determines from this analysis that it does not expect to receive cash flows sufficient to recover the entire amortized cost of the security, a credit loss exists, the impairment is considered other-than-temporary and is recognized in its consolidated statements of operations and comprehensive loss. 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. Warrants to Purchase Convertible Preferred Stock The Company accounts for warrant instruments that either conditionally or unconditionally obligate the issuer to transfer assets as liabilities regardless of the timing of the redemption feature or price, even though the underlying shares may be classified as equity. These warrants are subject to revaluation at each balance sheet date, and any changes in fair value are recorded as a component of other income/(expense), until the earlier of their exercise or expiration or the completion of a liquidation event, at which time the warrant liability may be reclassified to stockholders’ equity if the criteria for recording the warrant as an equity instrument are met. Per the terms of the warrants, upon completion of a qualified public offering, any unexercised warrants are converted into warrants to purchase common shares. 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 (new/used) 5 years / 3 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 such impairment charges during the years presented. Deferred Rent Certain of the Company’s operating lease agreements include scheduled rent escalations over the lease term, as well as lease incentives. Rent expense is charged ratably over the life of the lease. Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupies. Lease incentives are recorded as a deferred rent liability and are amortized on a straight-line basis over the term of the lease as a reduction to rent expense. 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. The unvested portion of the stock options is subject to re-measurement over the vesting period and forfeitures are recorded as they occur. 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 Attributable to Common Stockholders Basic net loss per share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Net loss applicable to common stockholders is calculated by adjusting the net loss of the Company for cumulative preferred stock dividends. Diluted net loss per share applicable to common stockholders 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, convertible preferred stock, warrants, stock options, 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. Recently Adopted Accounting Pronouncements In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-based Payment Accounting (“ASU 2016-09”). ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. Some of the areas of simplification apply only to non-public companies. This guidance was effective on December 31, 2016 for public entities. For entities other than public business entities, the amendments are effective for annual periods beginning after December 15, 2017, and interim periods within annual periods beginning after December 15, 2018. Early adoption is permitted for an entity in any interim or annual period for which financial statements have not been issued or made available for issuance. An entity that elects early adoption must adopt all amendments in the same period. The Company adopted ASU 2016-09 effective January 1, 2018 and the adoption of this guidance did not have a material impact on the Company’s consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230), Restricted Cash (“ASU 2016-18”). The amendments of ASU 2016-18 were issued to address the diversity in classification and presentation of changes in restricted cash and restricted cash equivalents on the statement of cash flows which is currently not addressed under Topic 230. ASU 2016-18 would require an entity to include amounts generally described as restricted cash and restricted cash equivalents with cash and cash equivalents when reconciling the beginning of period and end of period total amounts on the statement of cash flows. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2017, for public entities and no later than for annual reporting periods beginning after December 15, 2018, for non-public entities. Early adoption is permitted and the standard must be applied retrospectively. The Company adopted this standard as of January 1, 2017 retrospectively for all periods presented. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business (“ASU 2017-01”). This new standard clarifies the definition of a business and provides a screen to determine when an integrated set of assets and activities is not a business. The screen requires that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. Early adoption is permitted. The Company adopted the requirements of ASU 2017-01 as of January 1, 2016 and applied the screen when evaluating the nature of the assets received in connection with the acquisition of Epiva in 2016. As a result of applying this screen the Company concluded that Epiva was not a business. In May 2017, the FASB issued ASU 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2017, for both public entities and non-public entities. Early adoption is permitted. The Company adopted ASU 2016-09 during 2018 and the adoption of this guidance did not have a material impact on the Company’s consolidated financial statements. Accounting Pronouncements Issued and Not Adopted as of December 31, 2018 In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), and further updated through ASU 2016-12 (“ASU 2016-12”), which amends the existing accounting standards for revenue recognition. ASU 2014-09 is based on principles that govern the recognition of revenue at an amount to which an entity expects to be entitled when products are transferred to customers. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2017, for public entities and no later than for annual reporting periods beginning after December 15, 2018, for non-public entities. The new revenue standard may be applied retrospectively to each prior period presented or retrospectively with the cumulative effect recognized as of the date of adoption. The Company will adopt ASU 2014-09 on January 1, 2019 and has concluded the adoption will not have a material impact on its consolidated financial statements as the Company is not yet generating revenues. 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 standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2018, for public entities and no later than for annual reporting periods beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020 for non-public entities. Early adoption is permitted for all entities. ASU 2016-02 is expected to impact the Company’s consolidated financial statements as the Company has certain operating lease arrangements for which the Company is the lessee. Management is currently evaluating the impact the adoption of ASU 2016-02 will have on its consolidated financial statements and related disclosures. In November 2018, the FASB issued ASU No. 2018-18, “ Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606 ” (“ASU 2018-18”). The amendments in ASU 2018-18 make targeted improvements to generally accepted accounting principles (GAAP) for collaborative arrangements by clarifying that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. In addition, unit-of-account guidance in Topic 808 was aligned with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606. ASU 2018-18 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period. The amendments in this Update should be applied retrospectively to the date of initial application of Topic 606. The Company will adopt ASU 2018-18 on January 1, 2019 and has concluded the adoption will not have a material impact on its consolidated financial statements as the Company does not have any collaborative agreements under which any participant is considered a customer of the Company.

Investments

Investments	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Investments As of December 31, 2018 and 2017 , the Company had short-term investments, consisting entirely of U.S. treasury securities, of $54.8 million and none , respectively. The following table summarizes the Company's investments held at December 31, 2018 , which are all classified as available-for-sale (in thousands): Description Amortized Cost Unrealized Gain Unrealized Loss Fair Value December 31, 2018: U.S. treasury securities $ 54,836 $ — $ (18 ) $ 54,818 Total $ 54,836 $ — $ (18 ) $ 54,818 The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2018 the balance in the Company’s accumulated other comprehensive loss was comprised of activity related to the Company’s available-for-sale investments. There were no material realized gains or losses recognized on the sale or maturity of available-for-sale securities during the years ended December 31, 2018 or 2017 , and as a result, the Company did no t reclassify any amounts out of accumulated other comprehensive loss for the same periods. As of December 31, 2018 , the aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months was $54.8 million and none of these securities had remaining maturities of greater than one year. The Company has the intent and ability to hold such securities until recovery. The Company determined that there has been no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2018 or 2017 .

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 (in thousands): Description December 31, 2018 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 91,801 $ 91,801 $ — $ — U.S. treasury securities included within short-term investments $ 54,818 $ — $ 54,818 $ — Total $ 146,619 $ 91,801 $ 54,818 $ — Description December 31, 2017 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 25,042 $ 25,042 $ — $ — Total $ 25,042 $ 25,042 $ — $ — Liabilities: Preferred Stock Warrant Liability $ 424 $ — $ — $ 424 Total $ 424 $ — $ — $ 424 As of December 31, 2018 and 2017 , the Company's cash equivalents and short-term investments 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. Preferred Stock Warrant Liabilities (the Warrants) relate to warrants to purchase convertible preferred stock issued by the Company in connection with entering into debt facility transactions from 2015 through 2018 as well as assuming warrants to purchase convertible preferred stock in connection with the acquisition of Epiva, a privately held research company. These Warrants are considered a Level 3 liability since their fair value measurements are based, in part, on significant inputs not observed in the market and reflect the Company’s assumptions as to the fair value of the underlying convertible preferred stock and the expected volatility of the Company’s convertible preferred stock, as well as estimates regarding the number of shares for which the Warrants were exercisable. The estimated fair value of the Warrants was determined using the Black-Scholes option-pricing model. In periods that the instruments were outstanding, a significant input to the fair value of the Warrants was the fair value of the Series A Preferred Stock, Series A-1 Preferred Stock, Series A-3 Preferred Stock and Series B Preferred Stock. The fair value of the Warrants was remeasured at each reporting date using then-current assumptions with changes in fair value charged to other expense on the statements of operations and comprehensive loss. As of December 31, 2017 , the Warrants were valued at $0.4 million and included in other non-current liabilities on the consolidated balance sheets. The assumptions used represent the Company’s best estimates at the time of valuation. The following assumptions were used in valuing the material Warrants: December 31, 2017 Risk-free interest rate 2.3 - 2.4 % Expected dividend yield 0.00 % Expected term (in years) 7.9 - 8.6 Expected volatility 81 - 82 % Fair value of preferred stock $2.41 - 2.56 Based on the terms and conditions of the Company’s Warrants, upon closing of the Company’s IPO on May 11, 2018, the Warrants converted to common stock warrants. On that date, the Company performed the final remeasurement of the Warrants using the fair value of the underlying common shares of $16.00 per share on May 11, 2018, recorded the change in fair value in other income (expense), net in the consolidated statement of operations and comprehensive loss and reclassified the carrying value to additional paid-in capital. All outstanding warrants were exercised in full in the second quarter of 2018. The following assumptions were used to measure the fair market value of the Warrants at conversion on May 11, 2018. Risk-free interest rate 2.3 - 2.4 % Expected dividend yield 0.00 % Expected term (in years) 7.5 - 9.8 Expected volatility 81 - 82 % Fair value of common stock $ 16.00 In May 2018, all of the outstanding warrants, totaling 56,008 , were exercised for common stock at exercise prices ranging from $2.45 to $7.34 per share, of which, 8,599 shares were repurchased by the Company in a cashless transaction. The Company received net proceeds of $0.1 million in connection with the exercise of warrants settled in cash. As part of the February 2018 debt drawdown, the Company’s loan and security agreement was amended to include the payment of a $0.3 million success fee to the financial institution in the event of a liquidation event, including an initial public offering. The success fee represents an embedded derivative which the Company has bifurcated from the debt arrangement and carried at fair value. The debt derivative was initially considered a Level 3 liability, since the fair value measurements were based, in part, on significant inputs not observed in the market and reflected the Company’s best estimate of the probability of an IPO. As a result of the completion of the IPO in May, the Company remeasured the fair value of the success fee to the full payment amount. The following table provides a roll-forward of the fair value of the warrant and debt derivative liabilities measured at fair value on a recurring basis using Level 3 significant unobservable inputs (in thousands): Warrant Liability Debt Derivative Liability Balance at December 31, 2017 $ 424 $ — Issuance of embedded derivative instrument — 150 Change in fair value of derivative instrument included in other income (expense), net — 100 Settlement of derivative instrument — (250 ) Issuance of warrants to purchase convertible preferred stock 89 — Balance at Change in fair value of warrant liability, included in other income (expense), net 306 — Exercise of warrants (819 ) — Balance at December 31, 2018 $ — $ — The estimated fair value of long-term debt approximates its carrying value as the effective interest rate approximates market rates. The fair value of long-term debt, which may differ from its carrying value, is determined by market interest rates from debt arrangements which are observed in market trading which are similar to the Company’s arrangement and are considered a Level 2 input.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment consists of the following (in thousands): December 31, 2018 2017 Property and equipment: Lab equipment $ 5,393 $ 3,189 Leasehold improvements 1,824 1,334 Furniture and fixtures 525 217 Computers and software 115 77 Office equipment 9 9 Construction-in-process 1,011 99 Property and equipment 8,877 4,925 Less: accumulated depreciation (1,952 ) (1,429 ) Property and equipment, net $ 6,925 $ 3,496 The Company recognized $1.9 million , $0.8 million and $0.5 million of depreciation expense for the years ended December 31, 2018 , 2017 and 2016 , respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consists of the following (in thousands): December 31, 2018 2017 Accrued external research and development expenses $ 1,587 $ 715 Accrued payroll and related expenses 2,198 256 Accrued professional fees 1,010 1,081 Accrued other 170 147 Total accrued expenses $ 4,965 $ 2,199

Acquisition with Epiva Bioscien

Acquisition with Epiva Biosciences, Inc. (Epiva)	12 Months Ended
	Dec. 31, 2018
Asset Acquisition [Abstract]
Acquisition with Epiva Biosciences, Inc. (Epiva)	Acquisition with Epiva Biosciences, Inc. (Epiva) On June 16, 2016, the Company acquired Epiva, a privately held research company, focused on microbes for inflammatory diseases in order to create synergies and expand the depth of the Company’s research platform. Epiva held intellectual property rights related to microbes affecting the inflammatory diseases. The acquisition resulted in the exchange of all shares of Epiva stock for shares of the Company’s stock at an exchange rate of 1-for- 0.8333 . The holders of Epiva common stock and common stock options received shares of the Company’s common stock or options. The holders of Epiva Series A and A-2 Preferred Stock received shares of the Company’s Series A-1 and A-3 Preferred Stock, respectively. Both the Company and Epiva received funding from various investment funds that are managed by the same entity. The Company assessed the ownership structure of the two companies as well as the investment funds and determined, based on the ownership structure and other rights provided through other relevant arrangements, such as voting rights agreements, limited partnership agreements and general partnership agreements, that the ultimate controlling parent of each of the Company and Epiva was the same entity both immediately before and immediately after the acquisition. As a result, the Company and Epiva were considered to be under common control and the transaction was considered to be a related party transaction. The net assets received by the Company as a result of the acquisition were determined to represent an asset and not a business. This conclusion was primarily based on the fact that substantially all of the fair value of the gross assets received, excluding cash acquired, related to Epiva’s intellectual property rights. This conclusion considered the nature of Epiva’s operations immediately prior to the acquisition as well as Epiva’s limited operating history. As the acquisition was considered to represent an asset acquisition under common control, the assets and liabilities received were initially recorded by the Company at Epiva’s carrying value on the date of acquisition. The operations associated with the assets received from Epiva are presented within the statements of operations on a prospective basis from the date of the acquisition. Assets and liabilities received from Epiva as of June 16, 2016 (at the historical carrying value of Epiva) are as follows (in thousands): Assets: Cash and cash equivalents $ 10,411 Prepaid expenses and other current assets 156 Property and equipment, net 406 Other assets 71 Total assets $ 11,044 Liabilities: Accounts payable $ 438 Accrued expenses 74 Long-term debt, net of debt discount 2,923 Other noncurrent liabilities 64 Total liabilities $ 3,499

Loan and Security Agreement

Loan and Security Agreement	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Loan and Security Agreement	Loan and Security Agreement In November 2015, the Company entered into a loan and security agreement with a financial institution. The arrangement allowed the Company to borrow up to $4.0 million and, if certain criteria were met, to borrow up to an additional $1.5 million . The Company drew $4.0 million under the facility in the first half of 2016 and repaid these amounts in 2016. In connection with this arrangement, the Company issued a warrant that was originally exercisable into 100,000 shares of Series A Preferred Stock. The warrant was initially recorded at fair value and subsequently marked-to-market through the statements of operations. The issuance costs were expensed in 2016 upon the repayment of the loan. In connection with the 2016 acquisition of Epiva, the Company assumed Epiva’s credit facility (the “Credit Facility”) and the related $3.0 million of outstanding debt. Subsequent to the acquisition, the Company amended the Credit Facility to allow the Company to borrow up to $15.0 million , including the $3.0 million that was outstanding on the modification date and extending the maturity to August 15, 2020. During 2016, the Company borrowed an additional $7.0 million , bringing the total amounts outstanding as of December 31, 2016 and 2017 to $10.0 million . Under the terms of the Credit Facility the Company is required to make interest only payments through August 15, 2018. Upon the expiration of the interest only period, amounts borrowed will be repaid over 24 equal monthly payments of principal plus interest accrued through August 15, 2020. The amounts outstanding under the facility have an interest rate of the higher of (i) prime plus 0.25% or (ii) 3.75% per annum. The loan is secured by a lien on all Company assets, excluding intellectual property. In February 2018, the Company drew the additional $5.0 million available under the Credit Facility. This resulted in an increase to the interest rate to the higher of (i) prime plus 0.25% or (ii) 4.50% per annum. The interest only payment period was extended to August 15, 2019. Upon the expiration of the interest only period, amounts borrowed will be repaid over 24 equal monthly payments of principal plus interest accrued through August 15, 2021. As such, $2.5 million and $12.3 million of the debt obligation, net of debt discount, have been included as current and long-term, respectively, on the Company’s consolidated balance sheet. The Company was provided the ability to prepay the outstanding loan at its option with a prepayment fee of 2% of principal amount if prepayment was made before August 15, 2018 or 0.5% if the prepayment is made between August 15, 2018 and August 15, 2019. In conjunction with the February 2018 drawdown, the Company issued a warrant to purchase up to 34,722 shares of the Company’s Series B preferred stock at an exercise price of $1.80 per share. As part of the February 2018 drawdown, the loan and security agreement was amended to include the payment of a $0.3 million success fee to the financial institution in the event of a liquidation event, including an initial public offering. The success fee represented an embedded derivative which the Company bifurcated from the debt arrangement and carried at fair value. In May of 2018, the Company completed its IPO and paid the success fee of $0.3 million . In addition, the warrant issued in February of 2018 was exercised in May 2018. The Company has the following minimum aggregate future loan payments at December 31, 2018 (in thousands): 2019 $ 3,342 2020 8,021 2021 5,108 Total minimum payments $ 16,471 Less amounts representing interest and discount (1,666 ) Less current portion (2,500 ) Long-term debt, net of current portion $ 12,305 The Credit Facility contains 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 are no financial covenants associated with the agreement. The obligations under the agreement are subject to acceleration upon the occurrence of specified events of default, including a material adverse change in the Company’s business, operations or financial or other condition. The Company has determined that the risk of subjective acceleration under the material adverse events clause is remote and therefore has classified the outstanding principal in current and non-current liabilities based on scheduled principal payments. Interest expense related to the Company's Credit Facility for the years ended December 31, 2018 , 2017 and 2016 was $0.9 million , $0.5 million and $0.3 million , respectively.

In-License Agreements

In-License Agreements	12 Months Ended
	Dec. 31, 2018
Research and Development [Abstract]
In-License Agreements	n-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 Mayo Foundation for Medical Education and Research (“Mayo”). Under the 2016 Mayo License Agreement, Mayo was entitled to certain participation rights in connection with the issuance and sale of Series B Preferred Stock. The 2016 Mayo License Agreement allowed Mayo to purchase shares at the same price paid as other investors and is considered to be a fair value contract. In 2017 , Mayo purchased 1,666,667 shares of Series B Preferred Stock at $1.80 per share. Also pursuant to the 2016 Mayo License Agreement, Mayo received 490 shares of common stock upon the completion of certain project milestones as well as warrants to purchase common stock (the “Mayo Warrants”) exercisable for 18 shares and 116 shares of common stock upon the completion of certain additional project milestones. The Mayo Warrants were fully vested and expensed in 2016. On April 9, 2018, Mayo Foundation exercised its warrant and was issued 134 shares of common stock. On August 6, 2017, the Company and Mayo entered into a license agreement (“2017 Mayo License Agreement”). Under the 2017 Mayo License Agreement, Mayo granted the Company (i) an exclusive, worldwide, sublicensable license under Mayo’s rights to certain intellectual property and microbial strains (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 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 Mayo 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, 2018, 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. In addition, the Company also agreed to pay the University of Chicago a share of sublicense revenue. As of December 31, 2018, the Company has incurred milestone payments to date totaling approximately $0.4 million under the agreement of which approximately $0.1 million is currently due.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Lease Obligations 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 maintains an additional separate operating lease for office and laboratory space that is scheduled to expire in 2020. The leases require security deposits, which the Company has primarily met with letters of credit from a financial institution that are 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 is scheduled to expire in 2020. The future minimum rental payments to be received under this agreement total $0.7 million and are equivalent to the minimum payments due from the Company to the landlord. The Company recorded $3.2 million , $1.0 million and $0.5 million of rent expense for the years ended December 31, 2018 , 2017 and 2016 , respectively, which are net of sublease rental income of $0.2 million , none and none , respectively. The minimum aggregate future lease commitments at December 31, 2018 , are as follows (in thousands): 2019 $ 3,280 2020 3,088 2021 2,973 2022 3,062 2023 3,154 Thereafter 5,740 $ 21,297 Biose Industrie On February 15, 2018, the Company entered into an Exclusivity and Commitment Agreement with Biose Industrie (“Biose”), a French corporation, in which Biose has agreed to exclusively manufacture certain microbial biotherapeutic products for the Company and reserve 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 monoclonal microbials and for a set minimum number of manufacturing runs per year. Exclusivity fees paid and any minimum commitments are expensed as incurred. At December 31, 2018 , aggregate minimum payments over the remaining contract life total approximately $2.0 million . 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.

Stockholders_ Equity (Deficit)

Stockholders’ Equity (Deficit) and Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Stockholders’ Equity (Deficit) and Convertible Preferred Stock	Stockholders’ Equity (Deficit) and Convertible Preferred Stock Common Stock On April 27, 2018, the Company filed an amendment to its certificate of incorporation with the Secretary of State of the State of Delaware, to effect a 1-for-4.079 reverse stock split of the Company’s common stock. All share and per share data shown in the consolidated financial statements and related notes have been retroactively revised to reflect the reverse stock split. On May 11, 2018, the Company completed an IPO of 5,312,500 shares of its common stock for aggregate gross proceeds of $85.0 million . The Company received approximately $75.8 million in net proceeds after deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company. Upon closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 22,386,677 shares of common stock at the applicable conversion ratio then in effect On May 11, 2018, the Company filed a restated certificate of incorporation with the Secretary of the State of Delaware, which became effective in connection with the closing of the IPO. Pursuant to the restated certificate of incorporation, the Company is authorized to issue 200,000,000 shares of common stock and 10,000,000 shares of preferred stock. Convertible Preferred Stock Upon closing of the IPO in May 2018, all 91,315,295 outstanding shares of the Series A, Series A-1, Series A-2, Series A-3, Series B and Series C Preferred Stock automatically converted into 22,386,677 shares of the Company’s common stock at the applicable conversion ratio of 1-for-4.079. Prior to conversion, all shares of Preferred Stock accrued a cumulative dividend of 8% per annum. Dividends for the applicable periods are included in net loss attributable to common shareholders on the condensed consolidated statement of operations through the conversion date. All accrued dividends earned on Preferred Stock were forfeited as of the conversion. In February and March 2018, the Company issued a total of 25,232,199 shares of Series C Preferred Stock at purchase price of $3.23 for gross proceeds $81.5 million under the same terms as the Series B Preferred Stock. In 2017, the Company issued a total of 27,777,778 shares of Series B Preferred Stock at purchase price of $1.80 for gross proceeds $50.0 million in four separate closings in the first half of 2017. The terms of the Series B Preferred Stock modified certain terms of the existing Series A, A-1, A-2, and A-3 Preferred Stock. The amendments include removing certain redemption rights and allowing the Series B Preferred Stock to vote as part of the class of preferred stockholders. The amendments representing a modification of the Series A, Series A-1, Series A-2, and Series A-3 Preferred Stock. The Company concluded the modification did not result in incremental value to the shareholders and as such no incremental expense was recorded. Based on the removal of the redemption rights, the Company concluded that it was no longer probable that the Series A, Series A-1, Series A-2 and Series A-3 shares would become redeemable. As such, the Company ceased accreting these amounts to their redemption value each reporting period.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [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 allows 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, effective January 1, 2019, the number of shares authorized for issuance under the 2018 Incentive Plan was increased by 1,273,031 shares. 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 to four years. Stock options granted under the 2018 Plan expire no more than 10 years from the date of grant. As of December 31, 2018, equity-based incentive awards for the purchase up to 703,976 shares of the Company’s common stock have been issued under the 2018 Plan, of which none have been canceled or exercised. As of December 31, 2018 , 844,853 shares of common stock are available for future grant under the 2018 Plan, which includes 204,137 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 stock awards agreements, including vesting requirements, are 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 to four years. Stock options issued under the 2015 Plan expire no more than 10 years from the date of grant. As of the adoption of the 2018 Plan, the Company ceased making awards under the 2015 Plan. 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, 2018 , an aggregate of 5,758,518 options and other equity awards had been granted under the 2015 Plan, of which 904,550 have been exercised, 640,151 have been canceled and 18,468 have been repurchased as of December 31, 2018 . 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 included in the Company’s statements of operations is as follows (in thousands): 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, 2018 2017 2016 Research and development $ 2,508 $ 849 $ 205 General and administrative 3,551 693 214 Total stock-based compensation expense $ 6,059 $ 1,542 $ 419 Stock Options A summary of the Company’s stock option activity and related information is as follows: Shares Weighted Average - Exercise Price Weighted Average - Remaining Contractual Life Aggregate Intrinsic Value(1) (in thousands) Options outstanding at December 31, 2017 3,179,536 $ 1.88 9.05 $ 19,803 Granted 2,013,157 $ 11.27 Exercised (84,801 ) $ 0.83 Canceled (190,081 ) $ 4.56 Options outstanding at December 31, 2018 4,917,811 $ 5.64 8.52 $ 37,395 Exercisable at December 31, 2018 1,465,230 $ 1.57 7.63 $ 16,775 Vested and expected to vest as of December 31, 2018 (2) 4,917,811 $ 5.64 8.52 $ 37,395 (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. (2) This reflects the Company’s adoption and election under ASU 2016-9 to recognize forfeitures as they occur. The Company had 3,452,581 unvested stock options outstanding as of December 31, 2018 . The weighted-average fair value of options granted during the years ended December 31, 2018 , 2017 and 2016 was $8.00 , $4.89 and $1.06 , respectively. The aggregate intrinsic value of options exercised during the years ended December 31, 2018, 2017 and 2016 was $1.0 million , $0.8 million and $0.8 million , respectively. When utilizing the Black-Scholes option-pricing model to determine the grant date fair value of stock options granted to employees as well as the vesting or re-measurement date fair value for awards granted to non-employees, the Company used the following weighted average, or ranges of, assumptions for options granted to employees and options granted to non-employees: Employee option grants Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.74 % 2.03 % 1.33 % Expected life (in years) 6.17 6.18 5.66 Volatility 77.0 % 79.5 % 87.2 % Expected dividend rate 0.00 % 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 . Fair Value of Underlying Common Stock: Prior to the commencement of trading of the Company's common stock on the NASDAQ Global Select Market, or NASDAQ, on May 9, 2018 in connection with the Company's IPO, the Company determined the fair value of the underlying common stock based on input from management and approved by the Board of Directors, which utilized the valuation of the Company’s enterprise value determined utilizing various methods including the back-solve method, the option-pricing method, or OPM, or a hybrid of the probability-weighted expected return method, or PWERM, and the OPM. The total enterprise value was then allocated to the various outstanding equity instruments, including the underlying common stock, utilizing the option-pricing model. Following the Company's IPO, the fair value of the underlying common stock has been determined by referencing the closing price on the NASDAQ on the date of each award. Non-employee option grants Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.71 % 2.30 % 2.35 % Expected life (in years) 8.29 9.43 9.51 Volatility 75.6 % 78.9 % 89.0 % Expected dividend rate 0.00 % 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. On January 30, 2018, the Company issued 250,000 shares of Series B Preferred Stock to a non-employee consultant as part of the consideration for the service performed and completed in 2017. The Company recognized $0.7 million as general and administrative expense in the consolidated statement of operations of which $0.1 million was recorded in 2018. As of December 31, 2018 , total unrecognized stock-based compensation expense relating to unvested stock options was $18.7 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 3.10 years. Restricted Stock The Company permitted the early exercise of certain stock options prior to vesting by certain directors and officers. This practice ceased in 2017. Any shares issued pursuant to unvested options are restricted and subject to repurchase by the Company until the conditions for vesting are met. Accordingly, the Company has recorded the proceeds from the issuance of restricted stock as a liability in the consolidated balance sheets included as a component of other current and noncurrent liabilities based on the scheduled vesting dates. The amounts paid for shares purchased under an early exercise of stock options and subject to repurchase by the Company are reported in stockholders’ equity (deficit) once those shares vest. Upon termination of employment of an option holder, the Company has the right to repurchase, at the original purchase price, any unvested restricted shares. In 2016, there were 640,268 options exercised prior to vesting for total proceeds of $0.2 million to the Company. These exercises are not considered substantive for accounting purposes, and as such the related shares are treated as restricted share liabilities given the implicit repurchase feature. As of December 31, 2018 , the Company has recognized restricted stock liability of $0.1 million as other noncurrent liabilities. The Company reclassified $0.1 million to stockholders’ equity (deficit) upon vesting of restricted shares during each of the years ended December 31, 2018 , 2017 and 2016 , respectively. The remaining proceeds related to the unvested options of $0.1 million as of December 31, 2018 will be reclassified to stockholders’ equity (deficit) as the shares vest over a remaining weighted average vesting period of 1.18 years. A summary of restricted stock activity is as follows: Shares Weighted- Average Price Outstanding at December 31, 2017 257,876 $ 0.40 Vested (113,627 ) $ 0.32 Repurchased (18,468 ) $ 0.49 Outstanding at December 31, 2018 125,781 $ 0.45 As of December 31, 2018 , the Company had $0.3 million of unrecognized stock-based compensation expense related to its employee unvested restricted stock awards which is expected to be recognized over a remaining weighted average vesting period of 1.64 years. 2018 Employee Stock Purchase Plan The Company's board of directors adopted on April 18, 2018, and the Company’s stockholders approved, the 2018 Employee Stock Purchase Plan (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. Offering periods under the ESPP will commence when determined by the plan administrator. The Company has not begun any offering periods under the ESPP.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company has not recorded a tax provision for the periods presented due to the losses incurred and the need for a full valuation allowance on net deferred tax assets. 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 entirely in the United States: December 31, 2018 2017 U.S. federal tax statutory rate 21.0 % 34.0 % State taxes, net of federal benefit 6.5 % 5.6 % Non-deductible stock compensation (1.0 )% (1.0 )% Other non-deductible expenses (0.6 )% (0.5 )% Credits 2.3 % 0.8 % Change in federal tax rate due to tax reform — % (22.5 )% Change in valuation allowance (28.2 )% (16.5 )% Other — % 0.1 % — % — % December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 26,339 $ 13,183 Research and development credits 2,721 1,175 Capitalized research and development, patent and start-up costs 360 241 Accrued expenses 918 267 Stock based compensation 1,017 217 Depreciation (281 ) (66 ) Deferred tax assets before valuation allowance 31,074 15,017 Valuation allowance (31,074 ) (15,017 ) Net deferred tax assets $ — $ — On December 22, 2017, the Tax Cuts and Jobs Act (the Act) was enacted in the United States. The Act incorporates significant changes to U.S. corporate income tax laws including, among other things, a reduction in the statutory federal corporate income tax rate from 35% to 21% , an exemption for dividends received from certain foreign subsidiaries, a one-time repatriation tax on deemed repatriated earnings from foreign subsidiaries, eliminating the alternative minimum tax (AMT), immediate expensing of certain depreciable tangible assets, changing rules related to net operating loss carryforwards, and limitations on the deduction for net interest expense and certain executive compensation. In December 2017, the SEC issued SAB 118, which directs taxpayers to consider the impact of the U.S. legislation as “provisional” when it does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete its accounting for the change in tax law. The Company did not have any material changes to amounts previously recorded in 2017 as provisional under SAB 118. As of December 31, 2017 , as a result of the Act, we remeasured certain deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21% . The Company has no foreign earnings and therefore is not subject to transition tax. The amount recorded related to the remeasurement of our deferred tax balance was a $6.3 million provision that was offset by a valuation allowance. As of December 31, 2018 , the Company had approximately $98.0 million and $91.2 million in federal and state Net Operating Losses (“NOLs”), respectively. The Federal NOLs include $49.9 million which expire at various dates through 2037 and $48.0 million which carryforward indefinitely. The state NOLs expire at various dates through 2038 . As of December 31, 2018 , the Company had federal and state research credits of $2.1 million and $0.8 million , respectively, which expire at various dates through 2033 . 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 Internal Revenue Code provisions, 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 stock ownership, some of which may be outside the Company’s control. As a result, the Company’s ability to use 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 NOLs generated post tax reform will have an indefinite life, are not subject to carryback provisions and are limited to 80% of income in any year. 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, 2018 and 2017 , respectively. The valuation allowance increased by $16.1 million in 2018 , primarily due to increases in net operating losses and research and development credits. Management reevaluates the positive and negative evidence at each reporting period. As of December 31, 2018 and 2017 , 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, 2018
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, restricted common stock, convertible preferred stock and warrants to purchase convertible preferred stock, outstanding during the period determined using the if-converted and 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, 2018 2017 2016 Convertible preferred shares (as converted to common stock) — 16,139,518 9,329,570 Warrant to purchase convertible preferred shares (as converted to common stock) and common shares — 47,628 47,628 Unvested common stock from early exercise of options 125,781 257,876 412,796 Stock options to purchase common stock 4,917,811 3,179,536 2,127,261 Total 5,043,592 19,624,558 11,917,255

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions The Company entered into an employment agreement with Duncan McHale, an executive officer of Weatherden Ltd (“Weatherden”), a United Kingdom based clinical development consulting firm, as of December 15, 2017. Pursuant to the terms of the agreement, the Company has agreed to pay Mr. McHale £0.3 million per year to serve as the Company’s Chief Medical Officer. The Company receives clinical advisory services from Weatherden through a supply of service agreement that was entered into during 2017 and, subsequently, a master services agreement in 2018. Duncan McHale, the Company’s Chief Medical Officer is a part owner of Weatherden. During the years ended December 31, 2018 , 2017 and 2016 , the Company paid Weatherden $0.7 million , $0.3 million and $0.0 million , respectively. As of December 31, 2018 and 2017 , the amounts due to Weatherden were $0.2 million and $0.2 million , respectively. In June 2018, the Company entered into a subleasing arrangement with VL46, an affiliate of one of its stockholders, Flagship Venture Funds. Under the terms of the sublease, the Company will invoice VL46 for an aggregate $0.9 million in rent payments which are due during the period from July 1, 2018 through April 30, 2020 plus any related taxes and lease operating costs. As of December 31, 2018 , $0.2 million related to this sublease agreement has been recorded as an offset to rent expense within the consolidated statements of operations and comprehensive loss. In May 2014, the Company entered into a services agreement with Flagship Ventures Management, Inc., an affiliate of one of its stockholders, Flagship Venture Funds, to provide general and administrative services to the Company, including the employer portions of employee health and dental benefit plans for Evelo Biosciences employees and consulting services. The Company made payments under the agreement of $0.0 million , $0.0 million and $0.2 million during the years ended December 31, 2018 , 2017 and 2016 , respectively. As of December 31, 2018 , no amounts were due to Flagship Ventures Management, Inc. related to the services agreement.

Selected Quarterly Financial In

Selected Quarterly Financial Information	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information	Selected Quarterly Financial Information The following table contains quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. Three Months Ended 2018 (in thousands except per share data) March 31 June 30 September 30 December 31 Total operating expenses $ 10,425 $ 15,228 $ 16,457 $ 15,993 Total other expense (income), net 75 (82 ) (600 ) (550 ) Net loss $ (10,500 ) $ (15,146 ) $ (15,857 ) $ (15,443 ) Convertible preferred stock dividends (2,417 ) (1,520 ) — — Net loss attributable to common stockholders $ (12,917 ) $ (16,666 ) $ (15,857 ) $ (15,443 ) Net loss per share applicable to common stockholders, basic and diluted $ (3.29 ) $ (0.85 ) $ (0.50 ) $ (0.49 ) Three Months Ended 2017 (in thousands except per share data) March 31 June 30 September 30 December 31 Total operating expenses $ 5,208 $ 6,218 $ 8,038 $ 8,067 Total other expense, net 162 143 135 76 Net loss $ (5,370 ) $ (6,361 ) $ (8,173 ) $ (8,143 ) Convertible preferred stock dividends (1,225 ) (1,422 ) (1,708 ) (1,730 ) Net loss attributable to common stockholders $ (6,595 ) $ (7,783 ) $ (9,881 ) $ (9,873 ) Net loss per share applicable to common stockholders, basic and diluted $ (1.81 ) $ (2.09 ) $ (2.61 ) $ (2.57 )

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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 condensed 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 subsidiary, Evelo Biosciences Security Corporation. All intercompany transactions and balances of the subsidiary 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 this Annual Report on Form 10-K.
Emerging Growth Company Status	is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act of 2012, or 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 this offering 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; or has more than $700.0 million in market value of its stock held by non-affiliates (and it has been a public company for at least 12 months and has filed one annual report on Form 10-K); or it issues 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, cash equivalents and short-term investments. The Company places its cash, cash equivalents and short-term investments in primarily two custodian accounts at accredited financial institutions. The Company’s available-for-sale investments primarily consist of U.S. Treasury securities. The Company has not experienced any realized losses on any of its accounts and management believes such funds are subject to minimal credit risk. Such deposits have and will continue to exceed federally insured limits. As of December 31, 2018 and 2017 , 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 (deficit) that are excluded from 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. Cash equivalents are stated at cost, which approximates market value. The Company’s restricted cash consists of restricted cash in connection with building leases for the Company’s office and laboratory premises.
Investments	The Company accounts for and classifies its investments as either “available-for-sale,” “trading,” or “held-to-maturity,” in accordance with the accounting guidance related to the accounting and classification of certain investments in debt and equity securities. The determination of the appropriate classification is based primarily on management’s intent to sell the investment at the time of purchase. As of December 31, 2018 , all of the Company's investments were classified as available‑for‑sale securities. Available‑for‑sale securities are those securities which the Company views as available for use in current operations, if needed. The Company generally classifies its available‑for‑sale securities as short‑term investments, even though the stated maturity date may be one year or more beyond the current balance sheet date. Available‑for‑sale investments are stated at fair value with their unrealized gains and losses included in accumulated other comprehensive loss within stockholders’ (deficit) equity, until such gains and losses are realized in other income (expense) within the consolidated statements of operations and comprehensive loss or until an unrealized loss is considered other‑than‑temporary. The Company recognizes other‑than‑temporary impairments of its investments in debt securities when there is a decline in fair value below the amortized cost basis and if (a) it has the intent to sell the security or (b) it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. If either of these conditions is met, the Company recognizes the difference between the amortized cost of the security and its fair value at the impairment measurement date in the consolidated statements of operations and comprehensive loss. If neither of these conditions is met, the Company must perform additional analyses to evaluate whether the unrealized loss is associated with the creditworthiness of the issuer of the security rather than other factors, such as interest rates or market factors. If the Company determines from this analysis that it does not expect to receive cash flows sufficient to recover the entire amortized cost of the security, a credit loss exists, the impairment is considered other-than-temporary and is recognized in its consolidated statements of operations and comprehensive loss.
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.
Warrants to Purchase Convertible Preferred Stock	The Company accounts for warrant instruments that either conditionally or unconditionally obligate the issuer to transfer assets as liabilities regardless of the timing of the redemption feature or price, even though the underlying shares may be classified as equity. These warrants are subject to revaluation at each balance sheet date, and any changes in fair value are recorded as a component of other income/(expense), until the earlier of their exercise or expiration or the completion of a liquidation event, at which time the warrant liability may be reclassified to stockholders’ equity if the criteria for recording the warrant as an equity instrument are met. Per the terms of the warrants, upon completion of a qualified public offering, any unexercised warrants are converted into warrants to purchase common shares.
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 (new/used) 5 years / 3 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 such impairment charges during the years presented.
Deferred Rent	Certain of the Company’s operating lease agreements include scheduled rent escalations over the lease term, as well as lease incentives. Rent expense is charged ratably over the life of the lease. Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupies. Lease incentives are recorded as a deferred rent liability and are amortized on a straight-line basis over the term of the lease as a reduction to rent expense.
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. The unvested portion of the stock options is subject to re-measurement over the vesting period and forfeitures are recorded as they occur.
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 Attributable to Common Stockholders	Basic net loss per share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Net loss applicable to common stockholders is calculated by adjusting the net loss of the Company for cumulative preferred stock dividends. Diluted net loss per share applicable to common stockholders 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, convertible preferred stock, warrants, stock options, 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.
Recently Adopted and Recently Issued and Not Yet Adopted Accounting Pronouncements	In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-based Payment Accounting (“ASU 2016-09”). ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. Some of the areas of simplification apply only to non-public companies. This guidance was effective on December 31, 2016 for public entities. For entities other than public business entities, the amendments are effective for annual periods beginning after December 15, 2017, and interim periods within annual periods beginning after December 15, 2018. Early adoption is permitted for an entity in any interim or annual period for which financial statements have not been issued or made available for issuance. An entity that elects early adoption must adopt all amendments in the same period. The Company adopted ASU 2016-09 effective January 1, 2018 and the adoption of this guidance did not have a material impact on the Company’s consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230), Restricted Cash (“ASU 2016-18”). The amendments of ASU 2016-18 were issued to address the diversity in classification and presentation of changes in restricted cash and restricted cash equivalents on the statement of cash flows which is currently not addressed under Topic 230. ASU 2016-18 would require an entity to include amounts generally described as restricted cash and restricted cash equivalents with cash and cash equivalents when reconciling the beginning of period and end of period total amounts on the statement of cash flows. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2017, for public entities and no later than for annual reporting periods beginning after December 15, 2018, for non-public entities. Early adoption is permitted and the standard must be applied retrospectively. The Company adopted this standard as of January 1, 2017 retrospectively for all periods presented. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business (“ASU 2017-01”). This new standard clarifies the definition of a business and provides a screen to determine when an integrated set of assets and activities is not a business. The screen requires that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. Early adoption is permitted. The Company adopted the requirements of ASU 2017-01 as of January 1, 2016 and applied the screen when evaluating the nature of the assets received in connection with the acquisition of Epiva in 2016. As a result of applying this screen the Company concluded that Epiva was not a business. In May 2017, the FASB issued ASU 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2017, for both public entities and non-public entities. Early adoption is permitted. The Company adopted ASU 2016-09 during 2018 and the adoption of this guidance did not have a material impact on the Company’s consolidated financial statements. Accounting Pronouncements Issued and Not Adopted as of December 31, 2018 In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), and further updated through ASU 2016-12 (“ASU 2016-12”), which amends the existing accounting standards for revenue recognition. ASU 2014-09 is based on principles that govern the recognition of revenue at an amount to which an entity expects to be entitled when products are transferred to customers. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2017, for public entities and no later than for annual reporting periods beginning after December 15, 2018, for non-public entities. The new revenue standard may be applied retrospectively to each prior period presented or retrospectively with the cumulative effect recognized as of the date of adoption. The Company will adopt ASU 2014-09 on January 1, 2019 and has concluded the adoption will not have a material impact on its consolidated financial statements as the Company is not yet generating revenues. 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 standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. This guidance is effective for annual reporting periods, and interim periods within those years, beginning after December 15, 2018, for public entities and no later than for annual reporting periods beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020 for non-public entities. Early adoption is permitted for all entities. ASU 2016-02 is expected to impact the Company’s consolidated financial statements as the Company has certain operating lease arrangements for which the Company is the lessee. Management is currently evaluating the impact the adoption of ASU 2016-02 will have on its consolidated financial statements and related disclosures. In November 2018, the FASB issued ASU No. 2018-18, “ Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606 ” (“ASU 2018-18”). The amendments in ASU 2018-18 make targeted improvements to generally accepted accounting principles (GAAP) for collaborative arrangements by clarifying that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. In addition, unit-of-account guidance in Topic 808 was aligned with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606. ASU 2018-18 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period. The amendments in this Update should be applied retrospectively to the date of initial application of Topic 606. The Company will adopt ASU 2018-18 on January 1, 2019 and has concluded the adoption will not have a material impact on its consolidated financial statements as the Company does not have any collaborative agreements under which any participant is considered a customer of the Company.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted cash	The following reconciles cash, cash equivalents and restricted cash as of December 31, 2018 and 2017 , as presented on our statements of cash flows to their related balance sheet accounts (in thousands): December 31, 2018 2017 Cash and cash equivalents: Cash $ 1,300 $ 13,204 Money Market Funds 91,801 25,042 Total cash and cash equivalents 93,101 38,246 Restricted cash 1,250 1,500 Cash, cash equivalents and restricted cash $ 94,351 $ 39,746
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 (new/used) 5 years / 3 years Leasehold improvements Lesser of asset life or Property and equipment consists of the following (in thousands): December 31, 2018 2017 Property and equipment: Lab equipment $ 5,393 $ 3,189 Leasehold improvements 1,824 1,334 Furniture and fixtures 525 217 Computers and software 115 77 Office equipment 9 9 Construction-in-process 1,011 99 Property and equipment 8,877 4,925 Less: accumulated depreciation (1,952 ) (1,429 ) Property and equipment, net $ 6,925 $ 3,496

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Summary of Investments Classified as Available-For-Sale	The following table summarizes the Company's investments held at December 31, 2018 , which are all classified as available-for-sale (in thousands): Description Amortized Cost Unrealized Gain Unrealized Loss Fair Value December 31, 2018: U.S. treasury securities $ 54,836 $ — $ (18 ) $ 54,818 Total $ 54,836 $ — $ (18 ) $ 54,818

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 (in thousands): Description December 31, 2018 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 91,801 $ 91,801 $ — $ — U.S. treasury securities included within short-term investments $ 54,818 $ — $ 54,818 $ — Total $ 146,619 $ 91,801 $ 54,818 $ — Description December 31, 2017 Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 25,042 $ 25,042 $ — $ — Total $ 25,042 $ 25,042 $ — $ — Liabilities: Preferred Stock Warrant Liability $ 424 $ — $ — $ 424 Total $ 424 $ — $ — $ 424
Summary of Assumptions in Valuing Material Warrants	The following assumptions were used to measure the fair market value of the Warrants at conversion on May 11, 2018. Risk-free interest rate 2.3 - 2.4 % Expected dividend yield 0.00 % Expected term (in years) 7.5 - 9.8 Expected volatility 81 - 82 % Fair value of common stock $ 16.00 The following assumptions were used in valuing the material Warrants: December 31, 2017 Risk-free interest rate 2.3 - 2.4 % Expected dividend yield 0.00 % Expected term (in years) 7.9 - 8.6 Expected volatility 81 - 82 % Fair value of preferred stock $2.41 - 2.56
Roll-Forward of Fair Value of Warrant and Debt Derivative Liability Measured at Fair Value on a Recurring Basis	The following table provides a roll-forward of the fair value of the warrant and debt derivative liabilities measured at fair value on a recurring basis using Level 3 significant unobservable inputs (in thousands): Warrant Liability Debt Derivative Liability Balance at December 31, 2017 $ 424 $ — Issuance of embedded derivative instrument — 150 Change in fair value of derivative instrument included in other income (expense), net — 100 Settlement of derivative instrument — (250 ) Issuance of warrants to purchase convertible preferred stock 89 — Balance at Change in fair value of warrant liability, included in other income (expense), net 306 — Exercise of warrants (819 ) — Balance at December 31, 2018 $ — $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
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 (new/used) 5 years / 3 years Leasehold improvements Lesser of asset life or Property and equipment consists of the following (in thousands): December 31, 2018 2017 Property and equipment: Lab equipment $ 5,393 $ 3,189 Leasehold improvements 1,824 1,334 Furniture and fixtures 525 217 Computers and software 115 77 Office equipment 9 9 Construction-in-process 1,011 99 Property and equipment 8,877 4,925 Less: accumulated depreciation (1,952 ) (1,429 ) Property and equipment, net $ 6,925 $ 3,496

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consists of the following (in thousands): December 31, 2018 2017 Accrued external research and development expenses $ 1,587 $ 715 Accrued payroll and related expenses 2,198 256 Accrued professional fees 1,010 1,081 Accrued other 170 147 Total accrued expenses $ 4,965 $ 2,199

Acquisition with Epiva Biosci_2

Acquisition with Epiva Biosciences, Inc. (Epiva) (Tables)	12 Months Ended
	Dec. 31, 2018
Asset Acquisition [Abstract]
Schedule of Asset Acquisition	Assets and liabilities received from Epiva as of June 16, 2016 (at the historical carrying value of Epiva) are as follows (in thousands): Assets: Cash and cash equivalents $ 10,411 Prepaid expenses and other current assets 156 Property and equipment, net 406 Other assets 71 Total assets $ 11,044 Liabilities: Accounts payable $ 438 Accrued expenses 74 Long-term debt, net of debt discount 2,923 Other noncurrent liabilities 64 Total liabilities $ 3,499

Loan and Security Agreement (Ta

Loan and Security Agreement (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Minimum Aggregate Future Loan Payments	The Company has the following minimum aggregate future loan payments at December 31, 2018 (in thousands): 2019 $ 3,342 2020 8,021 2021 5,108 Total minimum payments $ 16,471 Less amounts representing interest and discount (1,666 ) Less current portion (2,500 ) Long-term debt, net of current portion $ 12,305

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Minimum Aggregate Future Lease Commitments	The minimum aggregate future lease commitments at December 31, 2018 , are as follows (in thousands): 2019 $ 3,280 2020 3,088 2021 2,973 2022 3,062 2023 3,154 Thereafter 5,740 $ 21,297

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [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, 2018 2017 2016 Research and development $ 2,508 $ 849 $ 205 General and administrative 3,551 693 214 Total stock-based compensation expense $ 6,059 $ 1,542 $ 419
Summary of Stock Option Activity	A summary of the Company’s stock option activity and related information is as follows: Shares Weighted Average - Exercise Price Weighted Average - Remaining Contractual Life Aggregate Intrinsic Value(1) (in thousands) Options outstanding at December 31, 2017 3,179,536 $ 1.88 9.05 $ 19,803 Granted 2,013,157 $ 11.27 Exercised (84,801 ) $ 0.83 Canceled (190,081 ) $ 4.56 Options outstanding at December 31, 2018 4,917,811 $ 5.64 8.52 $ 37,395 Exercisable at December 31, 2018 1,465,230 $ 1.57 7.63 $ 16,775 Vested and expected to vest as of December 31, 2018 (2) 4,917,811 $ 5.64 8.52 $ 37,395 (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. (2) This reflects the Company’s adoption and election under ASU 2016-9 to recognize forfeitures as they occur.
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 as well as the vesting or re-measurement date fair value for awards granted to non-employees, the Company used the following weighted average, or ranges of, assumptions for options granted to employees and options granted to non-employees: Employee option grants Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.74 % 2.03 % 1.33 % Expected life (in years) 6.17 6.18 5.66 Volatility 77.0 % 79.5 % 87.2 % Expected dividend rate 0.00 % 0.00 % 0.00 % Non-employee option grants Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.71 % 2.30 % 2.35 % Expected life (in years) 8.29 9.43 9.51 Volatility 75.6 % 78.9 % 89.0 % Expected dividend rate 0.00 % 0.00 % 0.00 %
Summary of Restricted Stock Activity	A summary of restricted stock activity is as follows: Shares Weighted- Average Price Outstanding at December 31, 2017 257,876 $ 0.40 Vested (113,627 ) $ 0.32 Repurchased (18,468 ) $ 0.49 Outstanding at December 31, 2018 125,781 $ 0.45

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	The Company’s loss before income tax for the periods presented was generated entirely in the United States: December 31, 2018 2017 U.S. federal tax statutory rate 21.0 % 34.0 % State taxes, net of federal benefit 6.5 % 5.6 % Non-deductible stock compensation (1.0 )% (1.0 )% Other non-deductible expenses (0.6 )% (0.5 )% Credits 2.3 % 0.8 % Change in federal tax rate due to tax reform — % (22.5 )% Change in valuation allowance (28.2 )% (16.5 )% Other — % 0.1 % — % — %
Schedule of Deferred Tax Assets	December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 26,339 $ 13,183 Research and development credits 2,721 1,175 Capitalized research and development, patent and start-up costs 360 241 Accrued expenses 918 267 Stock based compensation 1,017 217 Depreciation (281 ) (66 ) Deferred tax assets before valuation allowance 31,074 15,017 Valuation allowance (31,074 ) (15,017 ) Net deferred tax assets $ — $ —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 Convertible preferred shares (as converted to common stock) — 16,139,518 9,329,570 Warrant to purchase convertible preferred shares (as converted to common stock) and common shares — 47,628 47,628 Unvested common stock from early exercise of options 125,781 257,876 412,796 Stock options to purchase common stock 4,917,811 3,179,536 2,127,261 Total 5,043,592 19,624,558 11,917,255

Selected Quarterly Financial _2

Selected Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Selected Quarterly Financial Information	The operating results for any quarter are not necessarily indicative of results for any future period. Three Months Ended 2018 (in thousands except per share data) March 31 June 30 September 30 December 31 Total operating expenses $ 10,425 $ 15,228 $ 16,457 $ 15,993 Total other expense (income), net 75 (82 ) (600 ) (550 ) Net loss $ (10,500 ) $ (15,146 ) $ (15,857 ) $ (15,443 ) Convertible preferred stock dividends (2,417 ) (1,520 ) — — Net loss attributable to common stockholders $ (12,917 ) $ (16,666 ) $ (15,857 ) $ (15,443 ) Net loss per share applicable to common stockholders, basic and diluted $ (3.29 ) $ (0.85 ) $ (0.50 ) $ (0.49 ) Three Months Ended 2017 (in thousands except per share data) March 31 June 30 September 30 December 31 Total operating expenses $ 5,208 $ 6,218 $ 8,038 $ 8,067 Total other expense, net 162 143 135 76 Net loss $ (5,370 ) $ (6,361 ) $ (8,173 ) $ (8,143 ) Convertible preferred stock dividends (1,225 ) (1,422 ) (1,708 ) (1,730 ) Net loss attributable to common stockholders $ (6,595 ) $ (7,783 ) $ (9,881 ) $ (9,873 ) Net loss per share applicable to common stockholders, basic and diluted $ (1.81 ) $ (2.09 ) $ (2.61 ) $ (2.57 )

Organization and Basis of Pre_2

Organization and Basis of Presentation - Narrative (Details) $ in Thousands	May 11, 2018USD ($)shares	Apr. 27, 2018	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Gross proceeds from sale of common stock, convertible preferred stock and debt											$ 257,300
Cash, cash equivalents and short-term investments			$ 147,900								147,900
Net loss			15,443	$ 15,857	$ 15,146	$ 10,500	$ 8,143	$ 8,173	$ 6,361	$ 5,370	56,946	$ 28,047	$ 13,332
Accumulated deficit			$ 113,381				$ 56,411				$ 113,381	$ 56,411
Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Reverse stock split ratio		0.2452
Number of shares issued in transaction (in shares) | shares											5,312,500
Common Stock | IPO
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of shares issued in transaction (in shares) | shares	5,312,500
Aggregate gross proceeds from transaction	$ 85,000
Net proceeds from transaction	$ 75,800
Conversion of shares, shares converted (in shares) | shares	22,386,677

Significant Accounting Polici_4

Significant Accounting Policies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)segment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Accounting Policies [Abstract]
Comprehensive loss	$ 56,964	$ 28,047	$ 13,332
Restricted cash	$ 1,250	$ 1,500
Number of operating segments | segment	1

Significant Accounting Polici_5

Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and cash equivalents:
Cash	$ 1,300	$ 13,204
Money Market Funds	91,801	25,042
Total cash and cash equivalents	93,101	38,246
Restricted cash	1,250	1,500
Cash, cash equivalents and restricted cash	$ 94,351	$ 39,746	$ 15,786	$ 3,095

Significant Accounting Polici_6

Significant Accounting Policies - Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Computer hardware | Minimum
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	P3Y
Computer hardware | Maximum
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	P5Y
Computer software
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	P3Y
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	P7Y
Research and lab equipment, new equipment
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	P5Y
Research and lab equipment, old equipment
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	P3Y

Investments - Narrative (Detail

Investments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Short term investments	$ 54,818,000	$ 0
Realized gain (loss) on sale or maturity of available-for-sale securities	0	0
Reclassification out of other comprehensive loss	0	$ 0
Value of securities in unrealized loss position for less than twelve months	$ 54,800,000
Maturity period	1 year

Investments - Summary of Invest

Investments - Summary of Investments Classified as Available-For-Sale (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 54,836,000
Unrealized Gain	0
Unrealized Loss	(18,000)
Fair Value	54,818,000	$ 0
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	54,836,000
Unrealized Gain	0
Unrealized Loss	(18,000)
Fair Value	$ 54,818,000

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Total	$ 146,619	$ 25,042
Liabilities:
Financial liabilities		424
Preferred Stock Warrant Liability
Liabilities:
Financial liabilities		424
Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	91,801	25,042
U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure	54,818
Active Markets (Level 1)
Assets:
Total	91,801	25,042
Liabilities:
Financial liabilities		0
Active Markets (Level 1) | Preferred Stock Warrant Liability
Liabilities:
Financial liabilities		0
Active Markets (Level 1) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	91,801	25,042
Active Markets (Level 1) | U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure	0
Observable Inputs (Level 2)
Assets:
Total	54,818	0
Liabilities:
Financial liabilities		0
Observable Inputs (Level 2) | Preferred Stock Warrant Liability
Liabilities:
Financial liabilities		0
Observable Inputs (Level 2) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	0	0
Observable Inputs (Level 2) | U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure	54,818
Unobservable Inputs (Level 3)
Assets:
Total	0	0
Liabilities:
Financial liabilities		424
Unobservable Inputs (Level 3) | Preferred Stock Warrant Liability
Liabilities:
Financial liabilities		424
Unobservable Inputs (Level 3) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	0	$ 0
Unobservable Inputs (Level 3) | U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	May 31, 2018	May 11, 2018	Feb. 28, 2018	Dec. 31, 2017
Fair Value Disclosures [Line Items]
Value of warrants				$ 0.4
Outstanding warrants (in shares)	56,008
Number of shares repurchased (in shares)	8,599
Net proceeds from exercise of warrants	$ 0.1
Minimum
Fair Value Disclosures [Line Items]
Common stock exercise prices (in dollars per share)	$ 2.45
Maximum
Fair Value Disclosures [Line Items]
Common stock exercise prices (in dollars per share)	$ 7.34
Common Stock
Fair Value Disclosures [Line Items]
Fair value of stock (in dollars per share)		$ 16
Line of Credit
Fair Value Disclosures [Line Items]
Success fee	$ 0.3		$ 0.3

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Assumptions in Valuing Material Warrants (Details)	May 11, 2018$ / shares	Dec. 31, 2017$ / shares
Expected dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0	0
Fair value of preferred and common stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	16
Minimum | Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.023	0.023
Minimum | Expected term
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term (in years)	7 years 6 months	7 years 10 months 24 days
Minimum | Expected volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.81	0.81
Minimum | Fair value of preferred and common stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		2.41
Maximum | Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.024	0.024
Maximum | Expected term
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term (in years)	9 years 9 months 18 days	8 years 7 months 6 days
Maximum | Expected volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.82	0.82
Maximum | Fair value of preferred and common stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input		2.56

Fair Value Measurements - Roll-

Fair Value Measurements - Roll-Forward of Fair Value of Warrant And Debt Derivative Liability Measured at Fair Value on a Recurring Basis (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 424
Issuance of embedded derivative instrument and warrants to purchase convertible preferred stock	89
Change in fair value of derivative instrument and warrant liability included in other income (expense), net	306
Settlement of derivative instrument and exercise of warrants	(819)
Ending Balance	0
Debt Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0
Issuance of embedded derivative instrument and warrants to purchase convertible preferred stock	150
Change in fair value of derivative instrument and warrant liability included in other income (expense), net	100
Settlement of derivative instrument and exercise of warrants	(250)
Ending Balance	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property and equipment:
Property and equipment	$ 8,877	$ 4,925
Less: accumulated depreciation	(1,952)	(1,429)
Property and equipment, net	6,925	3,496
Lab equipment
Property and equipment:
Property and equipment	5,393	3,189
Leasehold improvements
Property and equipment:
Property and equipment	1,824	1,334
Furniture and fixtures
Property and equipment:
Property and equipment	525	217
Computers and software
Property and equipment:
Property and equipment	115	77
Office equipment
Property and equipment:
Property and equipment	9	9
Construction-in-process
Property and equipment:
Property and equipment	$ 1,011	$ 99

Property and Equipment, Net - N

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 1,935	$ 834	$ 495

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 1,587	$ 715
Accrued payroll and related expenses	2,198	256
Accrued professional fees	1,010	1,081
Accrued other	170	147
Total accrued expenses	$ 4,965	$ 2,199

Acquisition with Epiva Biosci_3

Acquisition with Epiva Biosciences, Inc. (Epiva) - Narrative (Details)	1 Months Ended
	Jun. 30, 2016
Epiva Biosciences, Inc.
Schedule of Asset Acquisition [Line Items]
Conversion ratio	0.8333

Acquisition with Epiva Biosci_4

Acquisition with Epiva Biosciences, Inc. (Epiva) - Schedule of Asset Acquisition (Details) - Epiva Biosciences, Inc. $ in Thousands	Jun. 16, 2016USD ($)
Assets
Cash and cash equivalents	$ 10,411
Prepaid expenses and other current assets	156
Property and equipment, net	406
Other assets	71
Total assets	11,044
Liabilities
Accounts payable	438
Accrued expenses	74
Long-term debt, net of debt discount	2,923
Other noncurrent liabilities	64
Total liabilities	$ 3,499

Loan and Security Agreement - N

Loan and Security Agreement - Narrative (Details)	1 Months Ended	12 Months Ended
	Feb. 28, 2018USD ($)$ / sharesshares	Dec. 31, 2018USD ($)payment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	May 31, 2018USD ($)	Jun. 30, 2016USD ($)	Jun. 16, 2016USD ($)	Nov. 30, 2015USD ($)shares
Line of Credit Facility [Line Items]
Loan and security agreement, borrowing capacity				$ 15,000,000				$ 4,000,000
Credit facility, additional borrowing capacity								$ 1,500,000
Borrowing under loan and security agreement, up to			$ 10,000,000	10,000,000		$ 4,000,000
Credit facility, additional amount borrowed	$ 5,000,000			7,000,000
Number of monthly payments | payment		24
Long-term portion of debt obligation		$ 12,305,000	9,966,000
Interest expense		900,000	$ 500,000	$ 300,000
Epiva Credit Facility
Line of Credit Facility [Line Items]
Borrowing under loan and security agreement, up to							$ 3,000,000
Line of Credit
Line of Credit Facility [Line Items]
Loan and security agreement, interest rate	4.50%		3.75%
Current portion of debt obligation		2,500,000
Long-term portion of debt obligation		$ 12,305,000
Prepayment fee percentage before August 15, 2018	2.00%
Prepayment fee percentage between August 15, 2018 and August 15, 2019	0.50%
Success fee	$ 300,000				$ 300,000
Line of Credit | Prime Plus
Line of Credit Facility [Line Items]
Loan and security agreement, basis spread on interest rate	0.25%		0.25%
Series A Preferred Stock
Line of Credit Facility [Line Items]
Number of shares called by warrants (in shares) | shares								100,000
Series B Preferred Stock
Line of Credit Facility [Line Items]
Number of shares called by warrants (in shares) | shares	34,722
Warrants exercise price per share (in dollars per share) | $ / shares	$ 1.8

Loan and Security Agreement - S

Loan and Security Agreement - Schedule of Minimum Aggregate Future Loan Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Long-term debt, net of current portion	$ 12,305	$ 9,966
Line of Credit
Line of Credit Facility [Line Items]
2,019	3,342
2,020	8,021
2,021	5,108
Total minimum payments	16,471
Less amounts representing interest and discount	(1,666)
Less current portion	(2,500)
Long-term debt, net of current portion	$ 12,305

In-License Agreements - Narrati

In-License Agreements - Narrative (Details) - USD ($)	Aug. 06, 2017	Mar. 10, 2016	Dec. 31, 2018	Apr. 09, 2018	Dec. 31, 2017	Jun. 10, 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Purchase of preferred stock (in shares)			0		0
Preferred stock par value per share (in dollars per share)			$ 0.001		$ 0.001
Common stock shares issued (in shares)			31,951,540		4,138,483
2016 Mayo License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Purchase of preferred stock (in shares)					1,666,667
Preferred stock par value per share (in dollars per share)					$ 1.8
Common stock shares issued (in shares)						490
Mayo Warrants
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Common stock shares issued (in shares)				134
Warrants to purchase of common stock exercisable (in shares)						18
Number of shares of common stock upon completion (in shares)						116
2017 Mayo License Agreement
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
2017 Mayo License Agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments upon achievement of certain development, regulatory, and commercial events	$ 55,960,000
2016 University of Chicago Agreement
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
Milestone payments for development and commercialization of licensed products			400,000
Amount due under license agreement			$ 100,000
2016 University of Chicago Agreement | 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)	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Jan. 31, 2018ft²
Commitments and Contingencies Disclosure [Abstract]
Area of leased office and research development space (in square feet) | ft²				40,765
Future minimum rental payments to be received	$ 700,000
Rent expense	3,200,000	$ 1,000,000	$ 500,000
Sublease rent income	200,000	$ 0	$ 0
Aggregate minimum payments due over remaining contract life	$ 2,000,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Minimum Aggregate Future Lease Commitments (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,019	$ 3,280
2,020	3,088
2,021	2,973
2,022	3,062
2,023	3,154
Thereafter	5,740
Aggregate future lease commitments	$ 21,297

Stockholders_ Equity (Deficit_2

Stockholders’ Equity (Deficit) and Convertible Preferred Stock - Narrative (Details) $ / shares in Units, $ in Millions	May 11, 2018USD ($)shares	Apr. 27, 2018	Mar. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2018shares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016shares	Dec. 31, 2015shares
Temporary Equity [Line Items]
Common stock, shares authorized (in shares)	200,000,000			200,000,000	23,780,338
Preferred stock, authorized (in shares)	10,000,000			10,000,000	0
Convertible preferred stock, shares outstanding (in shares)				0	65,833,096
Preferred stock dividend percentage	8.00%
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding (in shares)	91,315,295			0	65,833,096	38,055,318	12,536,945
Series C Preferred Stock
Temporary Equity [Line Items]
Preferred stock, issued (in shares)			25,232,199
Purchase price per share (in dollars per share) | $ / shares			$ 3.23
Gross proceeds | $			$ 81.5
Series B Preferred Stock
Temporary Equity [Line Items]
Preferred stock, issued (in shares)					27,777,778
Purchase price per share (in dollars per share) | $ / shares					$ 1.8
Gross proceeds | $					$ 50
Common Stock
Temporary Equity [Line Items]
Number of shares issued in transaction (in shares)				5,312,500
Reverse stock split ratio		0.2452
Common Stock | IPO
Temporary Equity [Line Items]
Number of shares issued in transaction (in shares)	5,312,500
Aggregate gross proceeds from transaction | $	$ 85
Net proceeds from transaction | $	$ 75.8
Conversion of shares, shares converted (in shares)	22,386,677

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2019	May 08, 2018	Jan. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of awards granted (in shares)				2,013,157
Number of shares canceled or exercised (in shares)				190,081
Unvested stock options outstanding (in shares)				3,452,581
Weighted average fair value of options granted (in dollars per share)				$ 8	$ 4.89	$ 1.06
Aggregate intrinsic value of options exercised				$ 1,000	$ 800	$ 800
Exercise of stock options (in shares)				84,801		640,268
Proceeds from stock options exercised						$ 200
Restricted stock liability, noncurrent				$ 100
Reclassification to stockholder's deficit				$ (136,949)	$ 54,723	$ 28,337	$ 4,471
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield				0.00%	0.00%	0.00%
Compensation cost not yet recognized, period for recognition				1 year 2 months 4 days
Reclassification to stockholder's deficit						$ (100)
Proceeds from unvested options				$ 100
Employee and Non-Employee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock based compensation expense, stock options				$ 18,700
Compensation cost not yet recognized, period for recognition				3 years 1 month 6 days
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost not yet recognized, period for recognition				1 year 7 months 22 days
Unrecognized stock based compensation expense, restricted stock				$ 300
Series B Preferred Stock | Non Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued in exchange for service (in shares)			250,000
Series B Preferred Stock | Non Employees | General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of underlying shares			$ 700	$ 100
2018 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of outstanding shares				4.00%
Number of shares canceled or exercised (in shares)				0
Number of common stock available for future grant (in shares)				844,853
2018 Stock Incentive Plan | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized for issuance (in shares)	1,273,031
2018 Stock Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of awards granted (in shares)				1,344,692
Number of shares issued for purchase (in shares)				703,976
2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares canceled or exercised (in shares)				204,137
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)				904,550
Number of shares, options and other equity awards canceled (in shares)				640,151
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 | Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
2015 Stock Incentive Plan | Minimum | Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				1 year
2015 Stock Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, maximum expiration period				10 years
2015 Stock Incentive Plan | Maximum | Non-Employee Options
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

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 6,059	$ 1,542	$ 419
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	2,508	849	205
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 3,551	$ 693	$ 214

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Options outstanding, beginning balance (in shares)	3,179,536
Granted (in shares)	2,013,157
Exercised (in shares)	(84,801)		(640,268)
Canceled (in shares)	(190,081)
Options outstanding, ending balance (in shares)	4,917,811	3,179,536
Exercisable at end of period (in shares)	1,465,230
Vested and expected to vest at end of period (in shares)	4,917,811
Weighted Average - Exercise Price
Options outstanding, beginning balance (in dollars per share)	$ 1.88
Granted (in dollars per share)	11.27
Exercised (in dollars per share)	0.83
Canceled (in dollars per share)	4.56
Options outstanding, ending balance (in dollars per share)	5.64	$ 1.88
Exercisable at end of period (in dollars per share)	1.57
Vested and expected to vest at end of period (in dollars per share)	$ 5.64
Weighted Average - Remaining Contractual Life
Options, outstanding, weighted average remaining contractual life	8 years 6 months 6 days	9 years 18 days
Options, exercisable, weighted average remaining contractual life	7 years 7 months 17 days
Options, vested and expected to vest, weighted average remaining contractual life	8 years 6 months 6 days
Options, outstanding, aggregate intrinsic value	$ 37,395	$ 19,803
Options, exercisable, aggregate intrinsic value	16,775
Options, vested and expected to vest, aggregate intrinsic value	$ 37,395

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock Option Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.74%	2.03%	1.33%
Expected life (in years)	6 years 2 months 1 day	6 years 2 months 5 days	5 years 7 months 28 days
Volatility	77.00%	79.50%	87.20%
Expected dividend rate	0.00%	0.00%	0.00%
Non-Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.71%	2.30%	2.35%
Expected life (in years)	8 years 3 months 14 days	9 years 5 months 5 days	9 years 6 months 5 days
Volatility	75.60%	78.90%	89.00%
Expected dividend rate	0.00%	0.00%	0.00%

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Shares
Outstanding restricted common stock, beginning balance (in shares) | shares	257,876
Vested (in shares) | shares	(113,627)
Repurchased (in shares) | shares	(18,468)
Outstanding restricted common stock, ending balance (in shares) | shares	125,781
Weighted- Average Exercise Price
Outstanding restricted common stock, beginning balance (in dollars per share) | $ / shares	$ 0.40
Vested (in dollars per share) | $ / shares	0.32
Repurchased (in dollars per share) | $ / shares	0.49
Outstanding restricted common stock, ending balance (in dollars per share) | $ / shares	$ 0.45

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. federal tax statutory rate	21.00%	34.00%
State taxes, net of federal benefit	6.50%	5.60%
Non-deductible stock compensation	(1.00%)	(1.00%)
Other non-deductible expenses	(0.60%)	(0.50%)
Credits	2.30%	0.80%
Change in federal tax rate due to tax reform	0.00%	(22.50%)
Change in valuation allowance	(28.20%)	(16.50%)
Other	0.00%	0.10%
Effective income tax rate reconciliation	0.00%	0.00%

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 26,339	$ 13,183
Research and development credits	2,721	1,175
Capitalized research and development, patent and start-up costs	360	241
Accrued expenses	918	267
Stock based compensation	1,017	217
Depreciation	(281)	(66)
Deferred tax assets before valuation allowance	31,074	15,017
Valuation allowance	(31,074)	(15,017)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Foreign earnings	$ 0
Tax cuts and jobs act of 2017, amount recorded related to remeasurement of deferred tax balance		$ 6,300,000
Income Tax Disclosure [Line Items]
Percentage of income limitation on carryback provisions	80.00%
Increase in valuation allowance	$ 16,100,000
Unrecognized tax benefits	0	$ 0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	98,000,000
Net operating loss carryforwards subject to expiration	49,900,000
Net operating loss carried forward indefinitely	48,000,000
Research credit carryforwards	2,100,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	91,200,000
Research credit carryforwards	$ 800,000

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	5,043,592	19,624,558	11,917,255
Warrant
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	0	47,628	47,628
Employee Stock Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	4,917,811	3,179,536	2,127,261
Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	0	16,139,518	9,329,570
Unvested Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	125,781	257,876	412,796

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) £ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2018GBP (£)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Duncan McHale, Chief Medical Officer
Related Party Transaction [Line Items]
Payment to related party | £		£ 0.3
Weatherden Ltd
Related Party Transaction [Line Items]
Payment to related party	$ 700,000		$ 300,000	$ 0
Amounts due to related party	200,000		200,000
VL46
Related Party Transaction [Line Items]
Rent due, sublease agreement	900,000
Revenue, sublease agreement	200,000
Flagship Ventures Management, Inc.
Related Party Transaction [Line Items]
Payment to related party	0		$ 0	$ 200,000
Amounts due to related party	$ 0

Selected Quarterly Financial _3

Selected Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses	$ 15,993	$ 16,457	$ 15,228	$ 10,425	$ 8,067	$ 8,038	$ 6,218	$ 5,208	$ 58,103	$ 27,531	$ 13,025
Total other expense (income), net	(550)	(600)	(82)	75	76	135	143	162	(1,157)	516	307
Net loss	(15,443)	(15,857)	(15,146)	(10,500)	(8,143)	(8,173)	(6,361)	(5,370)	(56,946)	(28,047)	(13,332)
Convertible preferred stock dividends	0	0	(1,520)	(2,417)	(1,730)	(1,708)	(1,422)	(1,225)	(3,937)	(6,085)	(1,645)
Net loss attributable to common stockholders	$ (15,443)	$ (15,857)	$ (16,666)	$ (12,917)	$ (9,873)	$ (9,881)	$ (7,783)	$ (6,595)	$ (60,883)	$ (34,132)	$ (14,977)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.49)	$ (0.50)	$ (0.85)	$ (3.29)	$ (2.57)	$ (2.61)	$ (2.09)	$ (1.81)	$ (2.78)	$ (9.10)	$ (5.28)