Evelo Biosciences (EVLO) 10-Q 3 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	Apr. 30, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	EVLO
Entity Registrant Name	Evelo Biosciences, Inc.
Entity Central Index Key	0001694665
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Emerging Growth Company	true
Small Business Entity	true
Entity Transition Period	false
Entity Common Stock, Shares Outstanding		32,043,577

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 105,490	$ 93,101
Short-term investments	23,949	54,818
Prepaid expenses and other current assets	3,843	3,703
Total current assets	133,282	151,622
Property and equipment, net	7,332	6,925
Other assets	1,320	1,320
Total assets	141,934	159,867
Current liabilities:
Accounts payable	1,379	1,519
Accrued expenses	5,237	4,965
Other current liabilities	4,600	2,751
Total current liabilities	11,216	9,235
Noncurrent liabilities:
Long-term debt, net of current portion	10,458	12,305
Deferred rent, net of current portion	1,097	1,071
Other noncurrent liabilities	280	307
Total liabilities	23,051	22,918
Commitments and contingencies
Stockholder’s equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued or outstanding at March 31, 2019 and December 31, 2018, respectively	0	0
Common stock, $0.001 par value; 200,000,000 shares authorized; 32,133,061 and 31,951,540 shares issued and 32,030,635 and 31,825,769 shares outstanding at March 31, 2019 and December 31, 2018, respectively	32	32
Additional paid-in capital	252,533	250,316
Accumulated other comprehensive loss	(2)	(18)
Accumulated deficit	(133,680)	(113,381)
Total stockholders’ equity	118,883	136,949
Total liabilities and stockholders’ equity	$ 141,934	$ 159,867

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	32,133,061	31,951,540
Common stock, shares outstanding (in shares)	32,030,635	31,825,769

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 15,680	$ 7,143
General and administrative	5,124	3,282
Total operating expenses	20,804	10,425
Loss from operations	(20,804)	(10,425)
Other income (expense):
Interest income (expense), net	505	46
Other expenses	0	(121)
Other income (expense), net	505	(75)
Net loss	(20,299)	(10,500)
Convertible preferred stock dividends	0	(2,417)
Net loss attributable to common stockholders	$ (20,299)	$ (12,917)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.64)	$ (3.29)
Weighted average number of common shares outstanding, basic and diluted (in shares)	31,925,072	3,922,152
Comprehensive loss:
Net loss	$ (20,299)	$ (10,500)
Other comprehensive loss:
Unrealized gain (loss) on investments, net of tax of $0	16	0
Comprehensive loss	$ (20,283)	$ (10,500)

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Tax portion of unrealized loss on investments	$ 0	$ 0

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Convertible Preferred Stock	Convertible Preferred Stock, Series B and C
Beginning balance (in shares) at Dec. 31, 2017						65,833,096
Beginning balance at Dec. 31, 2017						$ 83,702
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series B and Series C Preferred Stock, net of issuance costs (in shares)							25,482,199
Issuance of Series B and Series C Preferred Stock, net of issuance costs							$ 82,076
Ending balance (in shares) at Mar. 31, 2018						91,315,295
Ending balance at Mar. 31, 2018						$ 165,778
Beginning balance (in shares) at Dec. 31, 2017		3,880,607
Beginning balance at Dec. 31, 2017	$ (54,723)	$ 4	$ 1,708	$ 0	$ (56,435)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)		31,557
Vesting of restricted common stock	11		11
Exercise of stock options (in shares)		31,765
Exercise of stock options and warrants	35		35
Stock-based compensation expense	652		652
Unrealized loss on investments	0
Net loss	(10,500)				(10,500)
Ending balance (in shares) at Mar. 31, 2018		3,943,929
Ending balance at Mar. 31, 2018	(64,525)	$ 4	2,406	0	(66,935)
Beginning balance (in shares) at Dec. 31, 2018						0
Beginning balance at Dec. 31, 2018						$ 0
Ending balance (in shares) at Mar. 31, 2019						0
Ending balance at Mar. 31, 2019						$ 0
Beginning balance (in shares) at Dec. 31, 2018		31,825,769
Beginning balance at Dec. 31, 2018	136,949	$ 32	250,316	(18)	(113,381)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)		23,345
Vesting of restricted common stock	$ 7		7
Exercise of stock options (in shares)	181,521	181,521
Exercise of stock options and warrants	$ 257		257
Stock-based compensation expense	1,953		1,953
Unrealized loss on investments	16			16
Net loss	(20,299)				(20,299)
Ending balance (in shares) at Mar. 31, 2019		32,030,635
Ending balance at Mar. 31, 2019	$ 118,883	$ 32	$ 252,533	$ (2)	$ (133,680)

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating activities
Net loss	$ (20,299)	$ (10,500)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,953	652
Depreciation expense	375	585
Change in fair value of warrant and debt derivative liability	0	121
Net (accretion of discount)/amortization of premium on marketable securities	(115)	0
Non-cash interest expense	28	17
Changes in assets and liabilities:
Prepaid expenses and other current assets	(221)	(345)
Accounts payable	97	(44)
Accrued expenses and other current liabilities	185	653
Other liabilities	4	545
Net cash used in operating activities	(17,993)	(8,316)
Investing activities
Proceeds from sales and maturities of investments	31,000	0
Purchases of property and equipment	(876)	(1,000)
Net cash provided by/(used in) investing activities	30,124	(1,000)
Financing activities
Cash payment of initial public offering costs	0	(970)
Net proceeds from the issuance of convertible preferred stock	0	81,340
Net proceeds from the issuance of long-term debt	0	4,975
Proceeds from the exercise of stock options, restricted common stock and warrants	258	36
Net cash provided by financing activities	258	85,381
Net increase in cash, cash equivalents and restricted cash	12,389	76,065
Cash, cash equivalents and restricted cash – beginning of period	94,351	39,746
Cash, cash equivalents and restricted cash – end of period	106,740	115,811
Supplemental disclosure of cash flow information
Cash paid for interest	215	143
Noncash investing and financing activities
Property and equipment additions in accounts payable and accrued expenses	172	602
Issuance of warrants in connection with long-term debt facility	0	89
Accrued non-cash deferred offering costs	$ 0	$ 216

Organization

Organization	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	Organization Evelo Biosciences, Inc. ("Evelo" or the "Company”) is a biotechnology company which was incorporated in Delaware on May 6, 2014. The Company is discovering and developing potential therapies designed to engage immune cells in the small intestine and drive therapeutic effects throughout the body. The Company is advancing these monoclonal microbials with the aim of treating a broad range of immune mediated diseases with an initial focus on inflammatory diseases and oncology. The Company is headquartered in Cambridge, Massachusetts. Since inception, the Company has devoted substantially all of its efforts to research and development and raising capital. The Company has not generated any revenue related to its primary business purpose to date . The Company is subject to a number of risks similar to those of other development stage companies, including dependence on key individuals, the need to develop commercially viable products, competition from other companies, many of whom are larger and better capitalized, and the need to obtain adequate additional financing to fund the development of its products. The Company has funded its operations from the issuance of convertible notes, convertible preferred stock, common stock and debt financing. At March 31, 2019 , the Company had cash, cash equivalents and short-term investments of $129.4 million and an accumulated deficit of $133.7 million . The Company has incurred operating losses since inception and expects such losses and negative operating cash flows to continue for the foreseeable future. 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 March 31, 2019 will be sufficient to fund operations and capital expenditures for at least the twelve months following the filing of this Quarterly Report on Form 10-Q. 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited interim condensed 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”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018 and notes thereto. The unaudited interim consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of March 31, 2019 , the results of its operations and comprehensive loss for the three months ended March 31, 2019 and 2018 and cash flows for the three months ended March 31, 2019 and 2018 . Such adjustments are of a normal and recurring nature. The results for the three months ended March 31, 2019 are not necessarily indicative of the results for the year ending December 31, 2019 , or for any future period. 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 unaudited condensed 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. On April 1, 2019, the Company incorporated Evelo Biosciences (UK) Limited, its wholly owned subsidiary in London, England. 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 Quarterly Report on Form 10-Q. Emerging Growth Company Status Evelo is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act"), and it may take advantage of reduced reporting requirements that are otherwise applicable to public companies. Evelo may take advantage of these exemptions until it is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. Evelo has elected to use the extended transition period for complying with new or revised accounting standards, and, as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. Evelo may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its IPO or such earlier time that it is no longer an emerging growth company. Evelo would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, it has more than $700.0 million in market value of its stock held by non-affiliates (and has been a public company for at least 12 months and has filed one annual report on Form 10-K), or it has issued more than $1.0 billion of non-convertible debt securities over a three -year period. Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ equity that are excluded from net loss. The Company's only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments. Comprehensive loss totaled $20.3 million for the three months ended March 31, 2019 , and was not significantly different than net loss. For the three months ended March 31, 2018 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 and U.S. treasuries with original maturities of three months or less. 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 totaled approximately $1.3 million at both March 31, 2019 and December 31, 2018 and is classified within other assets on the accompanying condensed consolidated balance sheet. The following reconciles cash, cash equivalents and restricted cash as of March 31, 2019 and December 31, 2018 , as presented on the Company's statements of cash flows, to its related balance sheet accounts (in thousands): March 31, December 31, Cash and cash equivalents: Cash $ 953 $ 1,300 Money market funds 104,537 91,801 Total cash and cash equivalents 105,490 93,101 Restricted cash 1,250 1,250 Cash, cash equivalents and restricted cash $ 106,740 $ 94,351 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 March 31, 2019 , 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. 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. 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 purposes of allocating resources. Recently Adopted Accounting Pronouncements 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 adopted ASU 2014-09 on January 1, 2019 and has concluded that the adoption did not have a material impact on its consolidated financial statements as the Company is not yet generating revenues. 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 U.S. 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 adopted ASU 2018-18 on January 1, 2019 and has concluded the adoption did 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. Accounting Pronouncements Issued and Not Adopted as of March 31, 2019 In February 2016, the FASB issued ASU 2016-2, Leases (Topic 842) (“ASU 2016-2”), 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 most public entities. Early adoption is permitted for all entities. Although its assessment is not complete, the Company currently expects the adoption of this guidance to result in the addition of material balances of leased assets and corresponding lease liabilities to its consolidated balance sheets, primarily relating to leases of office space. In June 2018, the FASB issued ASU No. 2018-07, Stock-based Compensation: Improvements to Nonemployee Share-based Payment Accounting (Topic 718) ("ASU 2018-07"), which amends the existing accounting standards for share-based payments to nonemployees. This ASU aligns much of the guidance on measuring and classifying nonemployee awards with that of awards to employees. Under the new guidance, the measurement of nonemployee equity awards is fixed on the grant date. This ASU becomes effective in the first quarter of fiscal year 2020 and early adoption is permitted but no earlier than the Company's adoption date of Topic 606. Entities will apply the ASU by recognizing a cumulative-effect adjustment to retained earnings as of the beginning of the annual period of adoption. The Company is currently evaluating the impact that ASU 2018-07 will have on its consolidated financial statements.

Investments

Investments	3 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Investments	Investments As of March 31, 2019 and December 31, 2018 , the Company had short-term investments, consisting entirely of U.S. treasury securities, of $23.9 million and $54.8 million , respectively. The following tables summarize the Company's investments held at March 31, 2019 and December 31, 2018 , which are all classified as available-for-sale (in thousands): Description Amortized cost Unrealized Gain Unrealized Loss Fair Value March 31, 2019: U.S. treasury securities $ 23,951 $ — $ (2 ) $ 23,949 Total $ 23,951 $ — $ (2 ) $ 23,949 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 March 31, 2019 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 three months ended March 31, 2019 or 2018, and as a result, the Company did no t reclassify any amounts out of accumulated other comprehensive loss for the same periods. As of March 31, 2019 and December 31, 2018 , the aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months was $23.9 million and $54.8 million , respectively, and none of these securities had remaining maturities of greater than three years. 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 March 31, 2019 or December 31, 2018 .

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and December 31, 2018 (in thousands): Description March 31, Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 104,537 $ 104,537 $ — $ — U.S. treasury securities included within short-term investments 23,949 — 23,949 — Total $ 128,486 $ 104,537 $ 23,949 $ — Description December 31, 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 $ — As of March 31, 2019 and December 31, 2018 , 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.

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment consists of the following (in thousands): March 31, December 31, Property and equipment: Lab equipment $ 6,051 $ 5,393 Leasehold improvements 1,873 1,824 Furniture and fixtures 688 525 Computers and software 114 115 Office equipment 9 9 Construction-in-process 924 1,011 Property and equipment 9,659 8,877 Less: accumulated depreciation (2,327 ) (1,952 ) Property and equipment, net $ 7,332 $ 6,925 The Company recognized $0.4 million and $0.6 million of depreciation expense for the three months ended March 31, 2019 and 2018 , respectively.

Loan and Security Agreement

Loan and Security Agreement	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Loan and Security Agreement	Loan and Security Agreement The Company has a credit facility (the “Credit Facility”) with a bank that allows the Company to borrow up to $15.0 million . Borrowings under the Credit Facility are secured by a lien on all Company assets, excluding intellectual property. In February 2018, the Company drew the remaining $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 through 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, $4.4 million and $10.5 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 current portion of debt is included in other current liabilities. The Company has the ability to prepay the outstanding balance of the Credit Facility 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 March 31, 2019 (in thousands): Twelve month period ending March 31, Amount 2020 $ 5,175 2021 7,913 2022 3,170 Total minimum payments $ 16,258 Less amounts representing interest and discount (1,425 ) Less current portion (4,375 ) Long-term debt, net of current portion $ 10,458 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 long-term portion of the outstanding principal in non-current liabilities. Interest expense related to the Company's Credit Facility was approximately $0.2 million for both the three months ended March 31, 2019 and 2018 .

In-License Agreements

In-License Agreements	3 Months Ended
	Mar. 31, 2019
Research and Development [Abstract]
In-License Agreements	In-License Agreements Mayo Foundation for Medical Education and Research On June 10, 2016, the Company entered into a Research and License Agreement, (the “2016 Mayo License Agreement”) with Mayo Foundation for Medical Education and Research (“Mayo”). Under the 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 March 31, 2019 , 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. The University of Chicago maintains control of patent prosecution, defense and maintenance on their patent rights. The Company has the first right, but not obligation, to control any post-grant proceedings and to take action in the prosecution or prevention of any infringement by a third party to patent rights. The Company is responsible for reimbursing the University of Chicago, or directly paying, for patent prosecution, defense and/or maintenance costs incurred. This includes any costs associated with defense activities related to one of the patents underlying the 2016 University of Chicago Agreement and to which in April 2019 the United States Patent and Trademark Office instituted a post-grant review based on a petition from a third party. As of March 31, 2019 , the Company has incurred milestone payments to date totaling approximately $0.4 million .

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
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 $0.6 million and are equivalent to the minimum payments due from the Company to the landlord. The Company recorded rent expense of $0.7 million for each of the three months ended March 31, 2019 and 2018 . Rent expense for the three months ended March 31, 2019 is net of sublease rental income of $0.2 million . The minimum aggregate future lease commitments, exclusive of any offsetting sublease rental payments, at March 31, 2019 , are as follows (in thousands): Twelve month period ending March 31, Amount 2020 $ 3,304 2021 2,989 2022 2,995 2023 3,085 2024 3,178 Thereafter 4,934 $ 20,485 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. The Company has agreed to pay an exclusivity fee of $0.3 million per year. Litigation and Other Proceedings The Company may periodically become subject to legal proceedings and claims arising in connection with on-going business activities, including claims or disputes related to patents that have been issued or that are pending in the field of research on which the Company is focused. The Company is not a party to any material litigation and does not have contingency reserves established for any litigation liabilities. In April 2019, the United States Patent and Trademark Office ("USPTO"), granted a third party petition to initiate a post-grant review of a patent issued to the University of Chicago, to which the Company has an exclusive license from the University of Chicago. Although the Company believes that the subject patent is valid, there is a possibility that the USPTO could invalidate the patent or require the University of Chicago to narrow the claims contained in the patent. Under the terms of the license agreement the Company is responsible for reimbursing the University of Chicago for patent defense costs.

Stockholders_ Equity and Conver

Stockholders’ Equity and Convertible Preferred Stock	3 Months Ended
	Mar. 31, 2019
Temporary Equity Disclosure [Abstract]
Stockholders’ Equity and Convertible Preferred Stock	Stockholders’ Equity 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 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 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 of $81.5 million under the same terms as the Series B Preferred Stock.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2019
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 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 March 31, 2019 equity-based incentive awards for the purchase of up to 1,705,151 shares of the Company’s common stock have been issued under the 2018 Plan, of which none have been canceled or exercised. As of March 31, 2019 , 1,168,899 shares of common stock are available for future grant under the 2018 Plan, which includes 256,327 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 ("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 March 31, 2019 , an aggregate of 5,758,518 options and other equity awards had been granted under the 2015 Plan, of which 1,086,071 have been exercised and 692,336 have been canceled and 18,468 have been repurchased as of March 31, 2019 . A total of 113,006 shares previously reserved under the 2015 Plan that had not been exercised or were otherwise subject to outstanding exercise awards were no longer authorized as of May 8, 2018. Stock-Based Compensation Expense Stock-based compensation expense included in the Company’s condensed consolidated statements of operations and comprehensive loss is as follows (in thousands): Three Months Ended 2019 2018 General and administrative $ 1,062 $ 410 Research and development 891 242 Total stock-based compensation expense $ 1,953 $ 652 Stock Options A summary of the Company’s stock option activity and related information is as follows: Shares Weighted- Average Exercise Price Options outstanding at December 31, 2018 4,917,811 $ 5.64 Granted 1,001,175 13.00 Exercised (181,521 ) 1.42 Canceled (52,190 ) 4.47 Options outstanding at March 31, 2019 5,685,275 $ 7.09 Exercisable as of March 31, 2019 1,570,312 $ 2.36 The weighted-average fair value of options granted during the three months ended March 31, 2019 and 2018 was $8.82 and $7.91 , respectively. 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 services performed and completed in 2017. The Company has recognized $0.7 million as general and administrative expense in the condensed consolidated statement of operations and comprehensive loss, of which $0.1 million was recorded in the three months ended March 31, 2018 . As of March 31, 2019 , total unrecognized stock-based compensation expense relating to unvested stock options was $25.3 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 3.18 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 condensed 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. Shares Weighted- Average Exercise Price Outstanding and Unvested December 31, 2018 125,781 $ 0.45 Vested (23,345 ) 0.29 Outstanding and Unvested at March 31, 2019 102,436 $ 0.49 As of March 31, 2019 , the Company had $0.2 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 1.38 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	3 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. There were no significant income tax provisions or benefits for the three months ended March 31, 2019. Due to losses incurred since inception and the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2019
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: Three Months Ended 2019 2018 Convertible preferred shares (as converted to common stock) — 22,386,670 Warrant to purchase convertible preferred shares (as converted to common stock) — 56,008 Unvested common stock from early exercise of options 102,436 226,319 Stock options to purchase common stock 5,685,275 3,227,795 Total 5,787,711 25,896,792

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions The Company receives clinical advisory services from Weatherden Ltd. (“Weatherden”) under agreements that were entered into during 2017 and 2018. Duncan McHale, the Company’s Chief Medical Officer, is a part owner of Weatherden. During the three months ended March 31, 2019 and 2018 , the Company paid Weatherden $0.3 million and $0.2 million , respectively. As of March 31, 2019 and 2018 , the amounts due to Weatherden under the supply of service agreement were $0.1 million and $0.2 million , respectively. In June 2018, the Company entered into a subleasing arrangement with Ring Therapeutics (formerly VL46), an affiliate of one of its stockholders, Flagship Venture Funds. Under the terms of the sublease, the Company will invoice Ring Therapeutics 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 March 31, 2019 , $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.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited interim condensed 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”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018 and notes thereto. The unaudited interim consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of March 31, 2019 , the results of its operations and comprehensive loss for the three months ended March 31, 2019 and 2018 and cash flows for the three months ended March 31, 2019 and 2018 . Such adjustments are of a normal and recurring nature. The results for the three months ended March 31, 2019 are not necessarily indicative of the results for the year ending December 31, 2019 , or for any future period.
Use of Estimates	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	Principles of Consolidation The unaudited condensed 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	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 Quarterly Report on Form 10-Q.
Emerging Growth Company Status	Emerging Growth Company Status Evelo is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act"), and it may take advantage of reduced reporting requirements that are otherwise applicable to public companies. Evelo may take advantage of these exemptions until it is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. Evelo has elected to use the extended transition period for complying with new or revised accounting standards, and, as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. Evelo may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its IPO or such earlier time that it is no longer an emerging growth company. Evelo would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, it has more than $700.0 million in market value of its stock held by non-affiliates (and has been a public company for at least 12 months and has filed one annual report on Form 10-K), or it has issued more than $1.0 billion of non-convertible debt securities over a three -year period.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ equity that are excluded from net loss.
Cash, Cash Equivalents and Restricted Cash	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 and U.S. treasuries with original maturities of three months or less. 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	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 March 31, 2019 , 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	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.
Research and Development Costs	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.
Stock-Based Compensation	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	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 purposes of allocating resources.
Recently Adopted and Accounting Pronouncements Issued and Not Yet Adopted	Recently Adopted Accounting Pronouncements 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 adopted ASU 2014-09 on January 1, 2019 and has concluded that the adoption did not have a material impact on its consolidated financial statements as the Company is not yet generating revenues. 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 U.S. 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 adopted ASU 2018-18 on January 1, 2019 and has concluded the adoption did 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. Accounting Pronouncements Issued and Not Adopted as of March 31, 2019 In February 2016, the FASB issued ASU 2016-2, Leases (Topic 842) (“ASU 2016-2”), 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 most public entities. Early adoption is permitted for all entities. Although its assessment is not complete, the Company currently expects the adoption of this guidance to result in the addition of material balances of leased assets and corresponding lease liabilities to its consolidated balance sheets, primarily relating to leases of office space. In June 2018, the FASB issued ASU No. 2018-07, Stock-based Compensation: Improvements to Nonemployee Share-based Payment Accounting (Topic 718) ("ASU 2018-07"), which amends the existing accounting standards for share-based payments to nonemployees. This ASU aligns much of the guidance on measuring and classifying nonemployee awards with that of awards to employees. Under the new guidance, the measurement of nonemployee equity awards is fixed on the grant date. This ASU becomes effective in the first quarter of fiscal year 2020 and early adoption is permitted but no earlier than the Company's adoption date of Topic 606. Entities will apply the ASU by recognizing a cumulative-effect adjustment to retained earnings as of the beginning of the annual period of adoption. The Company is currently evaluating the impact that ASU 2018-07 will have on its consolidated financial statements.
Income Taxes	Income Taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following reconciles cash, cash equivalents and restricted cash as of March 31, 2019 and December 31, 2018 , as presented on the Company's statements of cash flows, to its related balance sheet accounts (in thousands): March 31, December 31, Cash and cash equivalents: Cash $ 953 $ 1,300 Money market funds 104,537 91,801 Total cash and cash equivalents 105,490 93,101 Restricted cash 1,250 1,250 Cash, cash equivalents and restricted cash $ 106,740 $ 94,351

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Summary of Short-Term Investments Classified as Available-For-Sale	The following tables summarize the Company's investments held at March 31, 2019 and December 31, 2018 , which are all classified as available-for-sale (in thousands): Description Amortized cost Unrealized Gain Unrealized Loss Fair Value March 31, 2019: U.S. treasury securities $ 23,951 $ — $ (2 ) $ 23,949 Total $ 23,951 $ — $ (2 ) $ 23,949 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)	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and December 31, 2018 (in thousands): Description March 31, Active Markets (Level 1) Observable Inputs (Level 2) Unobservable Inputs (Level 3) Assets: Money market funds included within cash and cash equivalents $ 104,537 $ 104,537 $ — $ — U.S. treasury securities included within short-term investments 23,949 — 23,949 — Total $ 128,486 $ 104,537 $ 23,949 $ — Description December 31, 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 $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment	Property and equipment consists of the following (in thousands): March 31, December 31, Property and equipment: Lab equipment $ 6,051 $ 5,393 Leasehold improvements 1,873 1,824 Furniture and fixtures 688 525 Computers and software 114 115 Office equipment 9 9 Construction-in-process 924 1,011 Property and equipment 9,659 8,877 Less: accumulated depreciation (2,327 ) (1,952 ) Property and equipment, net $ 7,332 $ 6,925

Loan and Security Agreement (Ta

Loan and Security Agreement (Tables)	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Schedule of Minimum Aggregate Future Loan Payments	The Company has the following minimum aggregate future loan payments at March 31, 2019 (in thousands): Twelve month period ending March 31, Amount 2020 $ 5,175 2021 7,913 2022 3,170 Total minimum payments $ 16,258 Less amounts representing interest and discount (1,425 ) Less current portion (4,375 ) Long-term debt, net of current portion $ 10,458

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Schedule of Minimum Aggregate Future Lease Commitments	The minimum aggregate future lease commitments, exclusive of any offsetting sublease rental payments, at March 31, 2019 , are as follows (in thousands): Twelve month period ending March 31, Amount 2020 $ 3,304 2021 2,989 2022 2,995 2023 3,085 2024 3,178 Thereafter 4,934 $ 20,485

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock-based Compensation Expense	Stock-based compensation expense included in the Company’s condensed consolidated statements of operations and comprehensive loss is as follows (in thousands): Three Months Ended 2019 2018 General and administrative $ 1,062 $ 410 Research and development 891 242 Total stock-based compensation expense $ 1,953 $ 652
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 Options outstanding at December 31, 2018 4,917,811 $ 5.64 Granted 1,001,175 13.00 Exercised (181,521 ) 1.42 Canceled (52,190 ) 4.47 Options outstanding at March 31, 2019 5,685,275 $ 7.09 Exercisable as of March 31, 2019 1,570,312 $ 2.36
Summary of Restricted Stock Activity	Upon termination of employment of an option holder, the Company has the right to repurchase, at the original purchase price, any unvested restricted shares. Shares Weighted- Average Exercise Price Outstanding and Unvested December 31, 2018 125,781 $ 0.45 Vested (23,345 ) 0.29 Outstanding and Unvested at March 31, 2019 102,436 $ 0.49

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
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: Three Months Ended 2019 2018 Convertible preferred shares (as converted to common stock) — 22,386,670 Warrant to purchase convertible preferred shares (as converted to common stock) — 56,008 Unvested common stock from early exercise of options 102,436 226,319 Stock options to purchase common stock 5,685,275 3,227,795 Total 5,787,711 25,896,792

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash, cash equivalents and short-term investments	$ 129,400
Accumulated deficit	$ (133,680)	$ (113,381)

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)segment	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)
Accounting Policies [Abstract]
Comprehensive loss	$ 20,283	$ 10,500
Restricted cash	$ 1,250		$ 1,250
Number of operating segments | segment	1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash and cash equivalents:
Cash	$ 953	$ 1,300
Money market funds	104,537	91,801
Total cash and cash equivalents	105,490	93,101
Restricted cash	1,250	1,250
Cash, cash equivalents and restricted cash	$ 106,740	$ 94,351	$ 115,811	$ 39,746

Investments - Narrative (Detail

Investments - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Short term investments	$ 23,949,000		$ 54,818,000
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	$ 23,900,000		54,800,000
Maturity period	3 years
U.S. treasury securities
Debt and Equity Securities, FV-NI [Line Items]
Short term investments	$ 23,949,000		$ 54,818,000

Investments - Summary of Short-

Investments - Summary of Short-Term Investments Classified as Available-For-Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 23,951	$ 54,836
Unrealized Gain	0	0
Unrealized Loss	(2)	(18)
Fair Value	23,949	54,818
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	23,951	54,836
Unrealized Gain	0	0
Unrealized Loss	(2)	(18)
Fair Value	$ 23,949	$ 54,818

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Total	$ 128,486	$ 146,619
Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	104,537	91,801
U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure	23,949	54,818
Active Markets (Level 1)
Assets:
Total	104,537	91,801
Active Markets (Level 1) | Money market funds included within cash and cash equivalents
Assets:
Cash and cash equivalents, fair value disclosure	104,537	91,801
Active Markets (Level 1) | U.S. treasury securities included within short-term investments
Assets:
Investments, fair value disclosure	0	0
Observable Inputs (Level 2)
Assets:
Total	23,949	54,818
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	23,949	54,818
Unobservable Inputs (Level 3)
Assets:
Total	0	0
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	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property and equipment:
Property and equipment	$ 9,659	$ 8,877
Less: accumulated depreciation	(2,327)	(1,952)
Property and equipment, net	7,332	6,925
Lab equipment
Property and equipment:
Property and equipment	6,051	5,393
Leasehold improvements
Property and equipment:
Property and equipment	1,873	1,824
Furniture and fixtures
Property and equipment:
Property and equipment	688	525
Computers and software
Property and equipment:
Property and equipment	114	115
Office equipment
Property and equipment:
Property and equipment	9	9
Construction-in-process
Property and equipment:
Property and equipment	$ 924	$ 1,011

Property and Equipment, Net - N

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation	$ 375	$ 585

Loan and Security Agreement - N

Loan and Security Agreement - Narrative (Details)	1 Months Ended	3 Months Ended
	Feb. 28, 2018USD ($)$ / sharesshares	Mar. 31, 2019USD ($)payment	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)	May 31, 2018USD ($)	Dec. 31, 2016USD ($)
Line of Credit Facility [Line Items]
Loan and security agreement, borrowing capacity						$ 15,000,000
Credit facility, additional amount borrowed	$ 5,000,000
Number of monthly payments | payment		24
Long-term portion of debt obligation		$ 10,458,000		$ 12,305,000
Interest expense		200,000	$ 200,000
Line of Credit
Line of Credit Facility [Line Items]
Loan and security agreement, interest rate	4.50%
Current portion of debt obligation		4,375,000
Long-term portion of debt obligation		$ 10,458,000
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%
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	$ 1.80

Loan and Security Agreement - S

Loan and Security Agreement - Schedule of Minimum Aggregate Future Loan Payments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Long-term debt, net of current portion	$ 10,458	$ 12,305
Line of Credit
Line of Credit Facility [Line Items]
2020	5,175
2021	7,913
2022	3,170
Total minimum payments	16,258
Less amounts representing interest and discount	(1,425)
Less current portion	(4,375)
Long-term debt, net of current portion	$ 10,458

In-License Agreements - Narrati

In-License Agreements - Narrative (Details) - USD ($)	Aug. 06, 2017	Mar. 10, 2016	Mar. 31, 2019	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)			32,133,061	31,951,540
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	$ 56,000,000
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
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) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Feb. 15, 2018USD ($)	Jan. 31, 2018ft²
Commitment And Contingencies [Line Items]
Area of leased office and research development space (in square feet) | ft²				40,765
Future minimum rental payments to be received	$ 0.6
Rent expense	0.7	$ 0.7
Sublease rent income	$ 0.2
Exclusivity and Commitment Agreement | Biose Industrie
Commitment And Contingencies [Line Items]
Exclusivity fee payable			$ 0.3

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Minimum Aggregate Future Lease Commitments (Details) $ in Thousands	Mar. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
2020	$ 3,304
2021	2,989
2022	2,995
2023	3,085
2024	3,178
Thereafter	4,934
Aggregate future lease commitments	$ 20,485

Stockholders_ Equity and Conv_2

Stockholders’ Equity and Convertible Preferred Stock - Narrative (Details) $ / shares in Units, $ in Millions	May 11, 2018USD ($)shares	Apr. 27, 2018	Mar. 31, 2018USD ($)$ / sharesshares	Mar. 31, 2019shares	Dec. 31, 2018shares	Dec. 31, 2017shares
Temporary Equity [Line Items]
Common stock, shares authorized (in shares)	200,000,000			200,000,000	200,000,000
Preferred stock, authorized (in shares)	10,000,000			10,000,000	10,000,000
Preferred stock dividend percentage	8.00%
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding (in shares)	91,315,295		91,315,295	0	0	65,833,096
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
Common Stock
Temporary Equity [Line Items]
Reverse stock split ratio		0.2451
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 Millions	Jan. 01, 2019	May 08, 2018	Apr. 18, 2018	Jan. 30, 2018	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares canceled or exercised (in shares)					52,190
Weighted average fair value of options granted (in dollars per share)					$ 8.82	$ 7.91
Employee and Non-Employee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock based compensation expense, stock options					$ 25.3
Compensation cost not yet recognized, period for recognition					3 years 2 months 4 days
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost not yet recognized, period for recognition					1 year 4 months 17 days
Unrecognized stock based compensation expense, restricted stock					$ 0.2
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				$ 0.7		$ 0.1
2018 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized for issuance (in shares)			336,356
Number of shares canceled or exercised (in shares)					0
Number of common stock available for future grant (in shares)					1,168,899
Percentage of outstanding shares			1.00%
2018 Stock Incentive Plan | Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2018 Stock Incentive Plan | Minimum | Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
2018 Stock Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares originally authorized for issuance (in shares)		1,344,692
Common stock outstanding percentage to be declared as an increase in grants					4.00%
Number of additional shares authorized for issuance (in shares)	1,273,031
Options granted, maximum expiration period					10 years
Number of shares issued for purchase (in shares)					1,705,151
2018 Stock Incentive Plan | Maximum | Non-Employee Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares originally authorized for issuance (in shares)		0
Number of shares canceled or exercised (in shares)					256,327
Number of common stock available for future grant (in shares)					5,417,044
Number of shares, options and other equity awards granted (in shares)					5,758,518
Number of shares, options and other equity awards exercised (in shares)					1,086,071
Number of shares, options and other equity awards canceled (in shares)					692,336
Number of shares 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

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,953	$ 652
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,062	410
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 891	$ 242

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Shares
Options outstanding, beginning balance (in shares) | shares	4,917,811
Granted (in shares) | shares	1,001,175
Exercised (in shares) | shares	(181,521)
Canceled (in shares) | shares	(52,190)
Options outstanding, ending balance (in shares) | shares	5,685,275
Exercisable as of period end (in shares) | shares	1,570,312
Weighted- Average Exercise Price
Options outstanding, beginning balance (in dollars per share) | $ / shares	$ 5.64
Granted (in dollars per share) | $ / shares	13
Exercised (in dollars per share) | $ / shares	1.42
Canceled (in dollars per share) | $ / shares	4.47
Options outstanding, ending balance (in dollars per share) | $ / shares	7.09
Exercisable as of period end (in dollars per share) | $ / shares	$ 2.36

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Shares
Outstanding and unvested, beginning balance (in shares) | shares	125,781
Vested (in shares) | shares	(23,345)
Outstanding and unvested, ending balance (in shares) | shares	102,436
Weighted- Average Exercise Price
Outstanding and unvested, beginning balance (in dollars per share) | $ / shares	$ 0.45
Vested (in dollars per share) | $ / shares	0.29
Outstanding and unvested, ending balance (in dollars per share) | $ / shares	$ 0.49

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	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	5,787,711	25,896,792
Warrant
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	0	56,008
Employee Stock Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	5,685,275	3,227,795
Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	0	22,386,670
Unvested Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	102,436	226,319

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Related Party Transaction [Line Items]
Rent due, sublease agreement	$ 0.6
Revenue, sublease agreement	0.2
Weatherden Ltd
Related Party Transaction [Line Items]
Payment to related party	0.3	$ 0.2
Amounts due to related party	0.1	$ 0.2
VL46
Related Party Transaction [Line Items]
Rent due, sublease agreement	0.9
Revenue, sublease agreement	$ 0.2