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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	May 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Trading Symbol	UBX
Entity Registrant Name	Unity Biotechnology, Inc.
Entity Central Index Key	0001463361
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		42,313,552

Condensed Balance Sheets (Unaudited) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 62,754,000	$ 7,298,000
Contribution receivable		1,382,000
Short-term marketable securities	71,281,000	79,212,000
Deferred offering costs	1,707,000	0
Prepaid expenses and other current assets	697,000	988,000
Total current assets	136,439,000	88,880,000
Property and equipment, net	6,591,000	6,958,000
Long-term marketable securities		5,118,000
Restricted cash	550,000	550,000
Other long-term assets	522,000	518,000
Total assets	144,102,000	102,024,000
Current liabilities:
Accounts payable	5,037,000	2,378,000
Accrued compensation	958,000	2,181,000
Accrued and other current liabilities	4,668,000	3,338,000
Total current liabilities	10,663,000	7,897,000
Deferred rent, net of current portion	3,062,000	3,166,000
Other non-current liabilities	101,000	118,000
Total liabilities	13,826,000	11,181,000
Commitments and contingencies (Note 6)
Convertible preferred stock, $0.0001 par value; 103,283,818 and 91,739,149 shares authorized as of March 31, 2018 and December 31, 2017, respectively; 31,750,297 and 28,159,724 shares issued and outstanding as of March 31, 2018 and December 31, 2017, respectively; aggregate liquidation preference of $245,825 as of March 31, 2018 and $190,825 as of December 31, 2017, respectively	228,907,000	173,956,000
Stockholders’ deficit:
Common stock, $0.0001 par value; 140,000,000 and 122,000,000 shares authorized as of March 31, 2018 and December 31, 2017, respectively; 5,230,976 and 4,830,389 shares issued and outstanding as of March 31, 2018 and December 31, 2017, respectively	1,000	1,000
Additional paid-in capital	5,511,000	4,072,000
Related party promissory notes for purchase of common stock	(592,000)	(202,000)
Employee promissory notes for purchase of common stock	(400,000)
Accumulated other comprehensive loss	(138,000)	(104,000)
Accumulated deficit	(103,013,000)	(86,880,000)
Total stockholders’ deficit	(98,631,000)	(83,113,000)
Total liabilities, convertible preferred stock, and stockholders’ deficit	$ 144,102,000	$ 102,024,000

Condensed Balance Sheets (Parenthetical) (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	103,283,818	91,739,149
Convertible preferred stock, shares issued	31,750,297	28,159,724
Convertible preferred stock, shares outstanding	31,750,297	28,159,724
Convertible preferred stock, liquidation preference	$ 245,825	$ 190,825
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	140,000,000	122,000,000
Common stock, shares issued	5,230,976	4,830,389
Common stock, shares outstanding	5,230,976	4,830,389

Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating expenses:
Research and development	$ 13,025	$ 6,970
General and administrative	3,457	2,070
Total operating expenses	16,482	9,040
Loss from operations	(16,482)	(9,040)
Interest income	352	108
Other expense, net	(3)	(2)
Net loss	(16,133)	(8,934)
Other comprehensive loss
Unrealized loss on marketable securities, net of tax	(34)
Comprehensive loss	$ (16,167)	$ (8,934)
Net loss per share, basic and diluted	$ (4.69)	$ (2.90)
Weighted average number of shares used in computing net loss per share, basic and diluted	3,437,345	3,079,551

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities
Net loss	$ (16,133)	$ (8,934)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	502	94
Amortization of premium and discounts on marketable securities	(33)	13
Stock-based compensation	1,370	413
Accretion of tenant improvement allowance	(152)	(151)
Changes in operating assets and liabilities:
Contribution receivable	1,382
Prepaid expenses and other current assets	291	(319)
Other long-term assets	(4)	26
Accounts payable	1,830	1,045
Accrued compensation	(1,223)	(190)
Accrued liabilities and other current liabilities	239	(66)
Deferred rent, net of current portion	48	209
Net cash used in operating activities	(11,883)	(7,860)
Investing activities
Purchase of marketable securities	(6,225)	(59,214)
Maturities of marketable securities	19,273
Purchase of property and equipment	(101)	(294)
Net cash provided by (used in) investing activities	12,947	(59,508)
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs	54,951	7,984
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	27	2
Payments of deferred offering costs	(569)
Payments made on capital lease obligations	(17)
Net cash provided by financing activities	54,392	7,986
Net increase (decrease) in cash, cash equivalents and restricted cash	55,456	(59,382)
Cash, cash equivalents and restricted cash at beginning of year	7,848	89,736
Cash, cash equivalents and restricted cash at end of year	63,304	30,354
Supplemental Disclosures of Non-Cash Investing and Financing Information
Property and equipment included in accounts payable	34	$ 126
Receipt of promissory note from related party for purchase of common stock	390
Receipt of promissory note from employees for purchase of common stock	400
Deferred offering costs included in accrued liabilities and accounts payable	$ 1,138

Organization and Liquidity Risks	3 Months Ended
Organization and Liquidity Risks	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Liquidity Risks	1. Organization Description of Business Unity Biotechnology, Inc. (the “Company” or “we” or “our” or “us”) is a biotechnology company engaged in the research and development of therapeutics to extend human healthspan. The Company devotes substantially all of its time and efforts to performing research and development, raising capital and recruiting personnel. The Company is located in Brisbane, California and was incorporated in the state of Delaware in March 2009 under the name Forge, Inc. and operates in one segment. The Company changed its name to Unity Biotechnology, Inc. in January 2015. Initial Public Offering On May 7, 2018, the Company closed its initial public offering (“IPO”), of 5,000,000 shares of common stock, at an offering price to the public of $17.00 per share. The Company received net proceeds of approximately $76.1 million, after deducting underwriting discounts, commissions and offering related transaction costs of approximately $8.9 million. In connection with the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 32,073,149 shares of common stock. In addition, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. In connection with the completion of its IPO, on May 7, 2018, the Company’s certificate of incorporation was amended and restated to provide for 300,000,000 authorized shares of common stock with a par value of $0.0001 per share and 10,000,000 authorized shares of preferred stock with a par value of $0.0001 per share.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

These financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) the rules and regulations of United States Securities and Exchange Commission (“SEC”) for interim reporting.

The condensed consolidated financial statements are unaudited and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, necessary for the fair presentation for interim reporting. The results of operations for any interim period are not necessarily indicative of results of operations for any future period. Certain information and footnote disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted. Accordingly, the unaudited condensed financial statements should be read in conjunction with the financial statements as of and for the year ended December 31, 2017, which are included in the Company’s prospectus related to the Company’s initial public offering, filed May 4, 2018 (the “Prospectus”), pursuant to Rule 424 (b) under the Securities Act of 1933, as amended with the SEC.

Reverse Stock Split

On April 19, 2018, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.95 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock, which was effected on April 20, 2018. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information included in the accompanying financial statements have been adjusted to reflect the Reverse Split. Accordingly, all share and per share information presented in the condensed consolidated financial statements herein, and notes thereto, have been retroactively adjusted to reflect the Reverse Split.

Need for Additional Capital

The Company has incurred operating losses and has an accumulated deficit as a result of ongoing efforts to develop drug product candidates, including conducting preclinical trials and providing general and administrative support for these operations. The Company had an accumulated deficit of $103.0 million and $86.9 million as of March 31, 2018 and December 31, 2017, respectively. The Company had net losses of $16.1 million and $8.9 million for the three months ended March 31, 2018 and 2017, respectively, and net cash used in operating activities of $11.9 million and $7.9 million for the three months ended March 31, 2018 and 2017, respectively. To date, none of the Company’s drug candidates have been approved for sale and therefore the Company has not generated any revenue from contracts with customers. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of one year following the date that these financial statements are issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of, or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.

Use of Estimates

The condensed financial statements have been prepared in accordance with GAAP, which requires management to make estimates and assumptions that affect the amounts and disclosures reported in the condensed financial statements and accompanying notes. The Company bases its estimates on historical experience and market-specific or other relevant assumptions that it believes are reasonable under the circumstances. The amounts of assets and liabilities reported in the Company’s balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining the fair value of assets and liabilities, common stock valuation, and stock-based compensation. Actual results could differ from such estimates or assumptions.

Deferred Offering Costs

Deferred offering costs, consisting of direct incremental legal, accounting, filing and other fees incurred related to the preparation of the IPO, have been capitalized and were offset against proceeds upon completion of the offering in May 2018. As of March 31, 2018, $1.7 million of deferred offering costs were capitalized and included in current assets on the balance sheet. There were no capitalized deferred offering costs at December 31, 2017.

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents primarily include money market funds that invest in U.S. Treasury obligations which are stated at fair value.

The Company has issued a letter of credit under a lease agreement which has been collateralized. This cash is classified as noncurrent restricted cash on the balance sheet based on the term of the underlying lease.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows.

	March 31,		December 31,
	2018		2017
	(in thousands)
Cash and cash equivalents	$	62,754	$	7,298
Restricted cash	550		550
Total cash, cash equivalents, and restricted cash	$	63,304	$	7,848

Variable Interest Entities

The Company reviews agreements it enters into with third-party entities, pursuant to which the Company may have a variable interest in the entity, in order to determine if the entity is a variable interest entity (“VIE”). If the entity is a VIE, the Company assesses whether or not it is the primary beneficiary of that entity. In determining whether the Company is the primary beneficiary of an entity, the Company applies a qualitative approach that determines whether it has both (i) the power to direct the economically significant activities of the entity and (ii) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. If the Company determines it is the primary beneficiary of a VIE, it consolidates that VIE into the Company’s financial statements. The Company’s determination about whether it should consolidate such VIEs is made continuously as changes to existing relationships or future transactions may result in a consolidation or deconsolidation event.

Concentrations of Risk

Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, marketable securities and contribution receivable. Substantially all of the Company’s cash and cash equivalents and restricted cash is deposited in accounts with financial institutions that management believes are of high credit quality. Such deposits have and will continue to exceed federally insured limits. The Company maintains its cash with accredited financial institutions and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses on its cash deposits. The contribution receivable is unsecured and is concentrated with one third-party organization, and accordingly the Company may be exposed to credit risk. To date, the Company has not experienced any loss related to its contributions receivable.

The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents, restricted cash and marketable securities and issuers of marketable securities to the extent recorded on the balance sheets. As of March 31, 2018, the Company had no off-balance sheet concentrations of credit risk.

The Company depends on third-party suppliers for key raw materials used in its manufacturing processes and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate raw materials.

Recently Issued Accounting Pronouncements

In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. This ASU clarifies the definition of a business when evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance is effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those years. For all other entities, it is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the effect that this guidance will have on its financial statements.

In February 2016, the FASB issued ASU No. 2016-02 (“ASU 2016-02”), Leases (Topic 842), 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. For public entities, this standard is effective for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period. For all other entities, this standard is effective for annual reporting periods beginning after December 15, 2019, and interim periods within annual periods beginning after December 15, 2020. Early adoption is permitted. The ASU is expected to impact the Company’s financial statements as the Company has certain operating lease arrangements for which the Company is the lessee. Management expects that the adoption of this standard will result in the recognition of an asset for the right to use the leased facility on the Company’s balance sheet, as well as the recognition of a liability for the lease payments remaining on the lease. While the Company is currently evaluating the impact of the adoption of this standard on its financial statements, the Company anticipates the recognition of additional assets and corresponding liabilities on its balance sheet related to leases.

In May 2017, the Financial Accounting Standards Board (FASB) issued ASU No. 2017-09, Compensation-Stock Compensation (Topic 718)- Scope of Modification Accounting (ASU 2017- 09). The amendments included in this update provide guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. The amendments in this update will be applied prospectively to an award modified on or after the adoption date. The amendments in ASU 2017-09 became effective for the Company on January 1, 2018 and the adoption of this standard did not have a material impact on the Company’s condensed financial statements.

In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. This guidance makes amendments to the classification and measurement of financial instruments and revises the accounting related to: (1) the classification and measurement of investments in equity securities (except for investments accounted for under the equity method of accounting); and (2) the presentation of certain fair value changes for financial liabilities measured at fair value. In addition, the update also amends certain disclosure requirements associated with the fair value of financial instruments. The guidance is effective for public business entities in 2018. For all other calendar-year entities, it is effective for annual periods beginning in 2019 and interim periods beginning in 2020. Early adoptions of certain amendments within the update are permitted. The Company is currently evaluating the impact that the adoption of this guidance will have on its financial statements and related disclosures, including on the Company’s cost method investment.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230: Classification of Certain Cash Receipts and Cash Payments). This guidance addresses specific cash flow issues with the objective of reducing the diversity in practice for the treatment of these issues. The areas identified include: debt prepayment or debt extinguishment costs; settlement of zero-coupon debt instruments; contingent consideration payments made after a business combination; proceeds from the settlement of insurance claims; proceeds from the settlement of corporate-owned life insurance policies; distributions received from equity method investees; beneficial interests in securitization transactions; and application of the predominance principle with respect to separately identifiable cash flows. The guidance will generally be applied retrospectively and is effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those years. For all other entities, it is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the effect that this guidance will have on its financial statements and related disclosures.

On December 22, 2017, the U.S. federal government enacted the Tax Cuts and Jobs Act (“the Act”). The Tax Act contains, among other things, significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of 21% for tax years beginning after December 31, 2017, limitation of the deduction for net operating losses to 80% of current year taxable income and elimination of net operating loss carrybacks, implementing a territorial tax system, and requiring a mandatory one-time tax on U.S. owned undistributed foreign earnings and profits known as the transition tax. In December 2017, SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”) to address the accounting implications of recently enacted U.S. federal tax reform. SAB 118 allows companies to record provisional amounts during a measurement period not to extend beyond one year of the enactment date to address ongoing guidance and tax interpretations that are expected over the next 12 months. The Company has adopted SAB 118 and currently considers its accounting of the impact of U.S. federal tax reform to be incomplete but continues to make a reasonable estimate of the effects on our existing deferred tax assets. The Company expects to complete the remainder of the analysis within the measurement period in accordance with SAB 118. Adjustments, if any, are not expected to impact the statement of operations and comprehensive loss due to the full valuation allowance on the Company’s deferred tax assets.

Fair Value Measurements

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The Company determines the fair value of financial and non-financial assets and liabilities based on the assumptions that market participants would use in pricing the asset or liability in an orderly transaction between market participants at the measurement date. The identification of market participant assumptions provides a basis for determining what inputs are to be used for pricing each asset or liability. A fair value hierarchy has been established which gives precedence to fair value measurements calculated using observable inputs over those using unobservable inputs. This hierarchy prioritized the inputs into three broad levels as follows:

•

Level 1: Quoted prices in active markets for identical instruments

•

Level 2: Other significant observable inputs (including quoted prices in active markets for similar instruments)

•

Level 3: Significant unobservable inputs (including assumptions in determining the fair value of certain investments)

The carrying amounts of financial instruments such as cash and cash equivalents, restricted cash, contributions receivable, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities approximate the related fair values due to the short maturities of these instruments.

The fair value of the Company’s cost method investment is measured when it is deemed to be other-than- temporarily impaired.

The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows:

	March 31, 2018
	Total		Level 1		Level 2		Level 3
	(in thousands)
Money market funds	$	62,085	$	62,085	$	—	$	—
Total	$	62,085	$	62,085	$	—	$	—
Short-term marketable securities:
Commercial paper		5,428		—		5,428		—
Corporate debt securities		10,140		—		10,140		—
Asset-backed securities		14,404		—		14,404		—
U.S. government debt securities		41,309		—		41,309		—
Total short-term marketable securities		71,281		—		71,281		—
Total	$	133,366	$	62,085	$	71,281	$	—

	December 31, 2017
	Total		Level 1		Level 2		Level 3
	(in thousands)
Money market funds	$	5,709	$	5,709	$	—	$	—
Total	$	5,709	$	5,709	$	—	$	—
Short-term marketable securities:
Commercial paper		6,359		—		6,359		—
Corporate debt securities		16,149		—		16,149		—
Asset-backed securities		14,588		—		14,588		—
U.S. government debt securities		42,116		—		42,116		—
Long-term marketable securities:
Asset-backed securities		2,742		—		2,742		—
U.S. government debt securities		2,376		—		2,376		—
Total marketable securities		84,330		—		84,330		—
Total	$	90,039	$	5,709	$	84,330	$	—

The Company estimates the fair value of its money market funds, commercial paper, corporate debt securities, asset-backed securities, and U.S. government debt securities by taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs.

There were no transfers within the hierarchy during the three months ended March 31, 2018 and the year ended December 31, 2017.

The grant date fair value of the Company’s common stock has been determined by the Company’s Board of Directors with the assistance of management and an independent third-party valuation specialist. The grant date fair value of the Company’s common stock was determined using valuation methodologies which utilizes certain assumptions including probability weighting of events, volatility, time to liquidation, a risk-free interest rate and an assumption for a discount for lack of marketability (Level 3 inputs). In determining the fair value of the Company’s common stock, the methodologies used to estimate the enterprise value of the Company were performed using methodologies, approaches, and assumptions consistent with the American Institute of Certified Public Accountants Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation (“AICPA Accounting and Valuation Guide”).

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Marketable Securities

3 Months Ended

Mar. 31, 2018

Investments Debt And Equity Securities [Abstract]

Marketable Securities

4. Marketable Securities

Marketable securities, which are classified as available-for-sale, consisted of the following as of March 31, 2018:


	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(in thousands)
Short-term marketable securities:
Commercial paper	$	5,445	$	—	$	(17	)	$	5,428
Corporate debt securities		10,167		—		(27	)		10,140
Asset-backed securities		14,446		—		(42	)		14,404
U.S. government debt securities		41,361		—		(52	)		41,309
Total short term marketable securities	$	71,419	$	—	$	(138	)	$	71,281

Marketable securities, which are classified as available-for-sale, consisted of the following as of December 31, 2017:


	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(in thousands)
Short-term marketable securities:
Commercial paper	$	6,369	$	—	$	(10	)	$	6,359
Corporate debt securities		16,162		—		(13	)		16,149
Asset-backed securities		14,604		—		(16	)		14,588
U.S. government debt securities		42,172		—		(56	)		42,116
Total short-term marketable securities		79,307		—		(95	)		79,212
Long-term marketable securities:
Asset-backed securities		2,752		—		(10	)		2,742
U.S. government debt securities		2,375		1		—			2,376
Total long-term marketable securities		5,127		1		(10	)		5,118
Total marketable securities	$	84,434	$	1	$	(105	)	$	84,330

At March 31, 2018, the remaining contractual maturities of available-for-sale securities were less than two years. There have been no significant realized gains or losses on available-for-sale securities for the periods presented. Available-for-sale debt securities that were in a continuous loss position but were not deemed to be other than temporarily impaired were immaterial at both March 31, 2018 and December 31, 2017.

License Agreements	3 Months Ended
License Agreements	Mar. 31, 2018
Research And Development [Abstract]
License Agreements	5. License Agreements License and Compound Library and Option Agreement In February 2016, the Company entered into a license agreement with a privately held clinical-stage biopharmaceutical company to research, develop, and seek and obtain marketing approval for a licensed compound. In February 2016, in conjunction with this license agreement, the Company also entered into a compound library and option agreement with the same biopharmaceutical company to identify compounds with potential utility in the treatment of age-related conditions other than indications in oncology. This license agreement included contingent consideration of up to 666,670 shares of additional common stock to be issued, up to $70.3 million of milestone payments based on achievement of certain specified clinical development and sales milestone events and tiered royalties in the low-single digits based on sales of licensed products. As of March 31, 2018 and 2017, none of the milestones had been achieved and no royalties were due from the sales of licensed products. In connection with the compound library and option agreement, the Company received an equity interest for 275,766 ordinary shares of an affiliate of the biopharmaceutical company at an aggregate purchase price of $0.5 million, which represents an insignificant level of ownership in the entity and approximates the fair value of the shares received and has been recorded as a cost method investments in the Company’s financial statements. The Company had a commitment to invest an additional $0.5 million in this entity in the future which was exercised in May 2018. The Company also agreed to provide funding to the biopharmaceutical company for research and development work performed at a cost of up to $2.0 million through February 2020. The research and development expense under the research services agreement was not material for the three months ended March 31, 2018 or 2017. Under the consolidation guidance, the Company determined that the biopharmaceutical company is a VIE. The Company does not have the power to direct the activities that most significantly affect the economic performance of this entity and as such the Company is not the primary beneficiary and consolidation is not required. As of March 31, 2018 and December 31, 2017, the Company has not provided financial, or other, support to the biopharmaceutical company that was not contractually required. Other License Agreements with Research Institutions The Company has entered into license agreements with various research institutions which have provided the Company with rights to patents, and in certain cases, research “know-how” and proprietary research tools to research, develop and commercialize drug candidates. In addition to upfront consideration paid to these various research institutions in either cash or shares of the Company’s common stock, the Company may be obligated to pay milestone payments in cash or the issuance of the Company’s common stock specific to each agreement on achievement of certain specified clinical development and/or sales events. To date, none of these events had occurred and no milestone or royalty payments have been recognized.

Commitments and Contingencies

3 Months Ended

Mar. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

Operating Lease

The Company has one non-cancelable operating leases consisting of administrative and research and development office space for its Brisbane, California headquarters that expires in October 2022. Future minimum lease payments under our non-cancellable operating leases at March 31, 2018 were as follows (in thousands):


	Amount
	(in thousands)
2018 (remaining 9 months)	$	1,473
2019		2,012
2020		2,072
2021		2,135
2022		1,621
Total future minimum lease payments	$	9,313

Rent expense for the three months ended March 31, 2018 and 2017 was $0.5 million and $0.5 million, respectively.

Indemnifications

The Company indemnifies each of its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with the Company’s amended and restated certificate of incorporation and bylaws. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity.

The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable the Company to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Related Party Transactions	3 Months Ended
Related Party Transactions	Mar. 31, 2018
Related Party Transactions [Abstract]
Related-Party Transactions	7. Related-Party Transactions Recourse Notes In December 2015, April 2016, and July 2016, the Company issued three full-recourse promissory notes to two executive officers for an aggregate principal amount of $0.2 million with an interest rate of 2.5% per annum. All of the principal was used to early exercise options for 667,253 shares of the Company’s common stock, in aggregate. All of these related party full-recourse notes were repaid on April 4, 2018 in accordance with the terms of such notes. In October 2017, the Company issued two promissory notes to an executive officer for $1.6 million and $0.5 million, each with an interest rate of 1.85% per annum. The aggregate principal amount of $2.1 million was used to purchase 625,084 shares of restricted stock. The promissory notes were considered to be non-recourse in substance and accordingly, the shares sold subject to such promissory notes are considered an option for accounting purposes. In April 2018, the Company’s board of directors approved the forgiveness of all outstanding principal and accrued interest of the $1.6 million non-recourse promissory note. The non-recourse promissory note outstanding of $0.5 million was repaid on April 4, 2018 in accordance with the terms of the note. In January 2018, the Company issued full-recourse promissory notes to an executive and an executive officer of the Company for an aggregate principal amount of $0.4 million with an interest rate of 2.5% per annum. All of the principal was used to early exercise options for 114,406 shares of the Company’s common stock. The full recourse note of $0.2 million for the executive officer was repaid on April 4, 2018 in accordance with the terms of the note. Financing Activities During the three months ended March 31, 2018, the Company issued convertible preferred stock for total proceeds of $3.0 million to shareholders who are considered to be related parties.

Convertible Preferred Stock and Common Stock

3 Months Ended

Mar. 31, 2018

Equity [Abstract]

Convertible Preferred Stock and Common Stock

8. Convertible Preferred Stock and Common Stock

Convertible Preferred Stock

In March 2018, the Company amended and restated its certificate of incorporation to, among other things, (i) increase its authorized shares of common stock from 122,000,000 to 140,000,000 shares, (ii) increase its authorized shares of preferred stock from 91,739,149 to 103,283,818 shares, of which 11,544,669 shares are designated as Series C convertible preferred stock, and (iii) set forth the rights, preferences and privileges of the Series C convertible preferred stock. In March 2018, the Company sold 3,590,573 shares of Series C convertible preferred stock at $15.3317 per share for net proceeds of $54.9 million and in April 2018, the Company sold an additional 322,852 shares of Series C convertible preferred stock $15.3317 per share for net proceeds of $5.0 million. .

Each share of Series C convertible preferred stock was convertible into one share of the Company’s common stock. Each share of preferred stock was automatically convert into one share of common stock upon the consummation of a qualified public offering. A qualified public offering was defined as an initial public offering that resulted in listing on a U.S. national securities exchange and at least $30.0 million of gross proceeds at a per share price of not less than the Series C original issue price of $15.3317.

The Company evaluated the other rights, preferences and privileges of each series of convertible preferred stock and concluded that there were either no freestanding derivative instruments or any embedded derivatives requiring bifurcation.

Convertible preferred stock consisted of the following:

	At March 31, 2018
	Shares Authorized		Shares Issued and Outstanding		Liquidation Preference		Carrying Value
	(in thousands, except for share amounts)
Series A-1		9,085,738		2,887,086	$	2,495	$	2,457
Series A-2		32,653,411		10,498,269		9,198		9,214
Series B		50,000,000		14,774,369		179,132		162,285
Series C		11,544,669		3,590,573		55,000		54,951
Total convertible preferred stock		103,283,818		31,750,297	$	245,825	$	228,907

	At December 31, 2017
	Shares Authorized		Shares Issued and Outstanding		Liquidation Preference		Carrying Value
	(in thousands, except for share amounts)
Series A-1		9,085,738		2,887,086	$	2,495	$	2,457
Series A-2		32,653,411		10,498,269		9,198		9,214
Series B		50,000,000		14,774,369		179,132		162,285
Total convertible preferred stock		91,739,149		28,159,724	$	190,825	$	173,956

Conversion Rights

Each share of convertible preferred stock was convertible at the right and option of the stockholder, at any time after the date of issuance, into such number of fully paid and non-assessable shares of common stock on a one for one ratio (1:1 conversion ratio). The Series A-1 conversion price was $0.864 per share, the Series A-2 conversion price was $0.876 per share, the Series B conversion price was $12.125 per share and the Series C conversion price was $15.3317, in each case, subject to certain antidilution adjustments as provided in the Company’s amended and restated certificate of incorporation.

Each share of convertible preferred stock was automatically convertible into a fully paid, non-assessable share of common stock at the then-effective conversion rate for such share (i) upon the closing of a firm commitment, underwritten initial public offering of the Company’s common stock at an aggregate gross proceeds of not less than $30.0 million and a price per share to the public of not less than $15.3317 per share; or (ii) upon the receipt by the Company of a written request for such conversion from at least 60% of the holders of the convertible preferred stock then outstanding (voting together as a single class and on an as-converted basis), or if later, the effective date for conversion specified in such requests.

Liquidation Rights

In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or a deemed liquidation event, as further defined in the Company’s amended and restated certificate of incorporation, prior to and in preference to any distribution of any of the assets of the Company to the holders of Series B convertible preferred stock and the Series A-1 and Series A-2 convertible preferred stock and common stock, the holders of Series C convertible preferred stock would have been paid, on a pari passu basis, an amount per share equal to the Series C liquidation preference of $15.3317 per share, plus an amount equal to any dividends declared but unpaid thereon (the “Series C Liquidation Preference”). If upon any such liquidation, dissolution or winding up of the Company or a deemed liquidation event, the assets of the Company available for distribution to its stockholders had been insufficient to pay the holders of Series C convertible preferred stock the full amount to which they were entitled, the holders of the Series C convertible preferred stock would have shared ratably in any distribution of the assets available for distribution in proportion to the respective amounts which would otherwise have been payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full.

After the payment or setting aside for payment to the holders of the Series C convertible preferred stock of the full amount of the Series C Liquidation Preference, prior to any distribution of any of the assets of the Company to the holders of the Series A-1 and Series A-2 convertible preferred stock and common stock, the holders of Series B convertible preferred stock would have been paid, on a pari passu basis, an amount per share equal to the Series B liquidation preference of $12.125 per share for Series B, plus, in each case, an amount equal to any dividends declared but unpaid thereon (the “Series B Liquidation Preference”). If upon any such liquidation, dissolution or winding up of the Company or deemed liquidation event, the assets of the Company available for distribution to its stockholders had been insufficient to pay the holders of shares of Series B convertible preferred stock the full amount to which they shall be entitled, the holders of the Series B convertible preferred stock would have shared ratably in any distribution of the assets available for distribution in proportion to the respective amounts which would otherwise have been payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full.

After the payment or setting aside for payment to the holders of the Series B convertible preferred stock of the full amount of the Series B Liquidation Preference, prior to any distribution of any of the assets of the Company to the holders of the common stock, the holders of Series A-1 and Series A-2 convertible preferred stock would have been paid, on a pari passu basis, an amount per share equal to $0.864 per share for Series A-1 and $0.876 per share for Series A-2, plus, in each case, an amount equal to any dividends declared but unpaid thereon (the “Series A Liquidation Preference”). If upon any such liquidation, dissolution or winding up of the Company or deemed liquidation event, the assets of the Company available for distribution to its stockholders had been insufficient to pay the holders of shares of Series A-1 and Series A-2 convertible preferred stock the full amount to which they shall be entitled, the holders of the Series A-1 and Series A-2 convertible preferred stock would have shared ratably in any distribution of the assets available for distribution in proportion to the respective amounts which would otherwise have been payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full.

After the payments or setting aside for payment to the holders of convertible preferred stock of the full amounts specified above, the entire remaining assets of the Company legally available for distribution shall be distributed pro rata to holders of the common stock of the Company in proportion to the number of shares of common stock held by them.

Voting Rights

The holders of outstanding shares of Series A-1 and Series A-2 convertible preferred stock, voting together as a single class, were entitled to elect two members of the Company’s Board of Directors. The holders of outstanding shares of Series B convertible preferred stock, voting together as a single class, were entitled to elect one member of the Company’s Board of Directors.

Additionally, each holder of the Company’s convertible preferred stock was entitled to a vote equal to the number of shares of common stock into which the shares of convertible preferred stock could have been converted as of the record date. The holders of convertible preferred were entitled to vote on all matters on which the common stock shall be entitled to vote.

Dividend Rights

Holders of the Series A-1, Series A-2, Series B and Series C convertible preferred stock were entitled to receive non-cumulative dividends at a rate of 6% of the original respective series of convertible preferred stock issuance price. Only after payment of the dividends to the holders of Series C convertible preferred stock were the holders of shares of Series B, Series A-1 and Series A-2 convertible preferred stock be entitled to receive dividends, out of any assets legally available therefore, prior and in preference to any declaration or payment of any dividend (other than dividends on the common stock payable solely in common stock) on the common stock.

After the payment or setting aside for payment of the dividends described above, any additional dividends (other than dividends on common stock payable solely in common stock) set aside or paid in any fiscal year could have been set aside or paid among the holders of the convertible preferred stock and common stock then outstanding on a pari passu basis in proportion to the greatest whole number of shares of common stock which would have been held by each such holder if all shares of convertible preferred stock were converted at the then-effective conversion rate.

Dividends were only payable as and if declared by the Board of Directors. To date, the Company has not declared or paid any dividends.

Redemption Rights

The convertible preferred stock was not mandatorily redeemable as it did not have a set redemption date or a date after which the shares may be redeemed by the holders. A redemption event would have occurred only upon the occurrence of certain change in control events that are outside the Company’s control, including a sale, lease, transfer, or other disposition of all or substantially all of the Company’s assets. The Company has elected not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because it is uncertain whether or when an event would have occurred that would obligate the Company to pay the liquidation preferences to holders of shares of convertible preferred stock. Subsequent adjustments to the carrying values of the liquidation preferences will be made only when it becomes probable that such a liquidation event will occur.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

9. Stock-Based Compensation

2018 Incentive Award Plan

In March 2018, the Company’s board of directors adopted the Company’s 2018 Incentive Award Plan (the “2018 Plan”). The 2018 Plan was approved by the Company’s stockholders in April 2018 and became effective on May 2, 2018. The 2018 Plan initially reserved 4,289,936 shares for the issuance of stock options as well as any automatic annual increases in the number of shares of common stock reserved for future issuance under the 2018 Plan. Awards granted under the 2018 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value on the day of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms.

2018 Employee Stock Purchase Plan

In March 2018, the Company’s board of directors adopted the Company’s 2018 Employee Stock Purchase Plan (the “2018 ESPP”). The 2018 ESPP was approved by the Company’s stockholders in April 2018 and became effective on May 2, 2018. The 2018 ESPP reserved 536,242 shares of common stock for issuance pursuant to future awards, as well as any automatic increases in the number of shares of our common stock reserved for future issuance under this plan.

2013 Equity Incentive Plan

In June 2013, the Company adopted the 2013 Equity Incentive Plan (the “2013 Plan”), which provides for the granting of incentive stock options (“ISOs”), non-statutory stock options (“NSOs”) and restricted shares to employees, directors, and consultants at the discretion of management and the Board of Directors. As of March 31, 2018, there were an aggregate of 450,233 shares of common stock authorized for issuance under the 2013 Plan.

The exercise price of an ISO and NSO shall not be less than 100% of the estimated fair value of the shares on the date of grant, and the exercise price of an ISO and NSO granted to a 10% stockholder shall not be less than 110% of the estimated fair value of the shares on the date of grant. For awards granted between September 2017 and March 2018 with an exercise price of $3.42, a deemed fair value ranging from $3.95 to $5.43 per share was used in calculating stock-based compensation expense, which was determined using management hindsight. Options granted under the 2013 Plan expire no later than 10 years from the date of grant and generally vest over a four-year period but may be granted with different vesting terms. The 2013Plan also provides that unvested options that were not exercised as of an employee’s termination date shall revert to the 2013 Plan.

Stock Option Activity

A summary of the Company’s stock option activity under the 2013 Plan is as follows:

	Number of Shares			Weighted- Average Exercise Price
Balances at December 31, 2017		4,196,203			3.06
Granted		493,776			5.79
Exercised		(400,587	)		3.10
Canceled		(25,423	)		3.39
Balances at March 31, 2018		4,263,969			3.37

Performance Contingent Stock Options Granted to Employees

During the three months ended March 31, 2018, the Board of Directors granted performance contingent stock option awards exercisable for 53,575 shares, to certain members of the Company’s executive team. These awards had a weighted average exercise price of $3.42 which was based on the fair market value on the grant date, as determined by the Board of Directors, and vest upon the successful achievement of one or more specified performance goals.

The total estimated fair value of employee performance contingent stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model using the same assumptions as the stock options granted to employees with service-based vesting conditions was $0.4 million. As of March 31, 2018, the Company determined that the achievement of the requisite performance conditions was not probable and, as a result, no compensation cost was recognized for the performance contingent awards.

As of March 31, 2018 and 2017, there were 329,498 and 275,922 performance contingent stock option awards outstanding with a total fair value of $0.7 million and $0.3 million respectively. As of March 31, 2018 and 2017, the Company determined that the achievement of the requisite performance conditions was not probable and, as a result, no compensation cost was recognized for these awards.

Performance and Market Contingent Stock Options Granted to Employees

During the three months ended March 31, 2018, the Board of Directors granted performance and market contingent stock option awards exercisable for 160,727 shares of common stock to certain members of the Company’s executive team. These awards had a weighted average exercise price of $3.42, which was based on the fair market value on the grant date, as determined by the Board of Directors. The total estimated grant-date fair value of these options was $0.7 million. Key assumptions in the valuation model included expected volatility, a risk-free interest rate, expected dividend yield, and an expected term unique to the terms of these awards.

Under the performance and market contingent awards, 53,575 of the shares have three separate market triggers for vesting based upon (i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum price per share, (ii) a change in control with aggregate proceeds payable to the Company’s common stock at a minimum price per share, or (iii) an initial public offering that becomes effective at a minimum specified price per share The remaining 107,152 shares have three separate market triggers for vesting based upon (i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum pre-money valuation, (ii) a change in control with a minimum aggregate proceeds payable to the Company’s common stock, or (iii) an initial public offering that becomes effective or an achievement of a minimum market capitalization, as measured by a trailing 30 day volume-weighted average price.

By definition, the market condition in these awards can only be achieved after the performance condition of a liquidity event has been achieved. As such, the requisite service period is based on the estimated period over which the market condition can be achieved. When a performance goal is deemed to be probable of achievement, time-based vesting and recognition of stock-based compensation expense commences. As of March 31, 2018, the Company determined that the achievement of the requisite performance conditions was not probable and, as a result, no compensation cost was recognized for these awards.

As of March 31, 2018 and 2017, there were 454,584 and 360,594 performance and market contingent stock option awards outstanding with a total fair value of $1.0 million and $0.4 million respectively. As of March 31, 2018 and 2017, the Company determined that the achievement of the requisite performance conditions was not probable and, as a result, no compensation cost was recognized for these awards.

Restricted Stock

A summary of the Company’s restricted stock activity for the three months ended March 31, 2018:

	Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2017		478,971		$	4.57
Granted		—		$	4.57
Vested		(119,742	)	$	4.57
Unvested at March 31, 2018		359,229		$	4.57

Stock-Based Compensation Expense

The following table sets forth the total stock-based compensation expense for all options granted to employees and nonemployees, including shares sold through the issuance of non-recourse promissory notes which are considered to be options for accounting purposes, included in the Company’s statement of operations:

	Three Months Ended March 31,
	2018		2017
	(in thousands)
Research and development	$	974	$	170
General and administrative		396		243
Total	$	1,370	$	413

Net Loss per Common Share

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Net Loss per Common Share

10. Net Loss per Common Share

Basic net loss per share is calculated by dividing net loss by the weighted average number of shares outstanding for the period. Diluted net loss per share is calculated by dividing net loss by the weighted average number of shares of common stock and potential dilutive common stock equivalents outstanding during the period if the effect is dilutive.

A reconciliation of the numerators and denominators used in computing net loss from continuing operations per share is as follows (in thousands, except per share amounts):

	March 31,
	2018			2017
Numerator:
Net loss	$	(16,133	)	$	(8,934	)
Denominator:
Weighted average number of shares outstanding—basic and diluted		3,437,345			3,079,551
Net loss per share—basic and diluted	$	(4.69	)	$	(2.90	)

Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:

	Three Months Ended March 31,
	2018		2017
Convertible preferred stock		31,750,297		25,280,436
Options to purchase common stock		4,433,459		2,332,221
Early exercised common stock subject to future vesting		1,058,270		1,199,098
Restricted stock accounted for as options		625,084		—
Warrants to purchase convertible preferred stock		763,501		763,501
Warrants to purchase common stock		96,610		96,610
Total		38,727,221		29,671,866

Up to 739,551 shares may be contingently issued, if certain performance conditions are met under our in-licensing agreements.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Reverse Stock Split

Need for Additional Capital

Use of Estimates

Deferred Offering Costs

Cash, Cash Equivalents and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows.

	March 31,		December 31,
	2018		2017
	(in thousands)
Cash and cash equivalents	$	62,754	$	7,298
Restricted cash	550		550
Total cash, cash equivalents, and restricted cash	$	63,304	$	7,848

Variable Interest Entities

Concentrations of Risk

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows.

	March 31,		December 31,
	2018		2017
	(in thousands)
Cash and cash equivalents	$	62,754	$	7,298
Restricted cash	550		550
Total cash, cash equivalents, and restricted cash	$	63,304	$	7,848

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Summary of Financial Assets Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Measurements

The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows:

	March 31, 2018
	Total		Level 1		Level 2		Level 3
	(in thousands)
Money market funds	$	62,085	$	62,085	$	—	$	—
Total	$	62,085	$	62,085	$	—	$	—
Short-term marketable securities:
Commercial paper		5,428		—		5,428		—
Corporate debt securities		10,140		—		10,140		—
Asset-backed securities		14,404		—		14,404		—
U.S. government debt securities		41,309		—		41,309		—
Total short-term marketable securities		71,281		—		71,281		—
Total	$	133,366	$	62,085	$	71,281	$	—

	December 31, 2017
	Total		Level 1		Level 2		Level 3
	(in thousands)
Money market funds	$	5,709	$	5,709	$	—	$	—
Total	$	5,709	$	5,709	$	—	$	—
Short-term marketable securities:
Commercial paper		6,359		—		6,359		—
Corporate debt securities		16,149		—		16,149		—
Asset-backed securities		14,588		—		14,588		—
U.S. government debt securities		42,116		—		42,116		—
Long-term marketable securities:
Asset-backed securities		2,742		—		2,742		—
U.S. government debt securities		2,376		—		2,376		—
Total marketable securities		84,330		—		84,330		—
Total	$	90,039	$	5,709	$	84,330	$	—

Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Marketable Securities Classified as Available-for-Sale

Marketable securities, which are classified as available-for-sale, consisted of the following as of March 31, 2018:


	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(in thousands)
Short-term marketable securities:
Commercial paper	$	5,445	$	—	$	(17	)	$	5,428
Corporate debt securities		10,167		—		(27	)		10,140
Asset-backed securities		14,446		—		(42	)		14,404
U.S. government debt securities		41,361		—		(52	)		41,309
Total short term marketable securities	$	71,419	$	—	$	(138	)	$	71,281

Marketable securities, which are classified as available-for-sale, consisted of the following as of December 31, 2017:


	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(in thousands)
Short-term marketable securities:
Commercial paper	$	6,369	$	—	$	(10	)	$	6,359
Corporate debt securities		16,162		—		(13	)		16,149
Asset-backed securities		14,604		—		(16	)		14,588
U.S. government debt securities		42,172		—		(56	)		42,116
Total short-term marketable securities		79,307		—		(95	)		79,212
Long-term marketable securities:
Asset-backed securities		2,752		—		(10	)		2,742
U.S. government debt securities		2,375		1		—			2,376
Total long-term marketable securities		5,127		1		(10	)		5,118
Total marketable securities	$	84,434	$	1	$	(105	)	$	84,330

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Summary of Future Minimum Lease Payments under Non-cancellable Operating Leases


	Amount
	(in thousands)
2018 (remaining 9 months)	$	1,473
2019		2,012
2020		2,072
2021		2,135
2022		1,621
Total future minimum lease payments	$	9,313

Convertible Preferred Stock and Common Stock (Tables)

3 Months Ended

Mar. 31, 2018

Equity [Abstract]

Summary of Convertible Preferred Stock

Convertible preferred stock consisted of the following:

	At March 31, 2018
	Shares Authorized		Shares Issued and Outstanding		Liquidation Preference		Carrying Value
	(in thousands, except for share amounts)
Series A-1		9,085,738		2,887,086	$	2,495	$	2,457
Series A-2		32,653,411		10,498,269		9,198		9,214
Series B		50,000,000		14,774,369		179,132		162,285
Series C		11,544,669		3,590,573		55,000		54,951
Total convertible preferred stock		103,283,818		31,750,297	$	245,825	$	228,907

	At December 31, 2017
	Shares Authorized		Shares Issued and Outstanding		Liquidation Preference		Carrying Value
	(in thousands, except for share amounts)
Series A-1		9,085,738		2,887,086	$	2,495	$	2,457
Series A-2		32,653,411		10,498,269		9,198		9,214
Series B		50,000,000		14,774,369		179,132		162,285
Total convertible preferred stock		91,739,149		28,159,724	$	190,825	$	173,956

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

A summary of the Company’s stock option activity under the 2013 Plan is as follows:

	Number of Shares			Weighted- Average Exercise Price
Balances at December 31, 2017		4,196,203			3.06
Granted		493,776			5.79
Exercised		(400,587	)		3.10
Canceled		(25,423	)		3.39
Balances at March 31, 2018		4,263,969			3.37

Summary of Restricted Stock Activity

A summary of the Company’s restricted stock activity for the three months ended March 31, 2018:

	Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2017		478,971		$	4.57
Granted		—		$	4.57
Vested		(119,742	)	$	4.57
Unvested at March 31, 2018		359,229		$	4.57

Summary of Stock-based Compensation Expense

	Three Months Ended March 31,
	2018		2017
	(in thousands)
Research and development	$	974	$	170
General and administrative		396		243
Total	$	1,370	$	413

Net Loss per Common Share (Tables)

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Schedule of Reconciliation of Numerators and Denominators Used In Computing Net Loss From Continuing Operations Per Share

A reconciliation of the numerators and denominators used in computing net loss from continuing operations per share is as follows (in thousands, except per share amounts):

	March 31,
	2018			2017
Numerator:
Net loss	$	(16,133	)	$	(8,934	)
Denominator:
Weighted average number of shares outstanding—basic and diluted		3,437,345			3,079,551
Net loss per share—basic and diluted	$	(4.69	)	$	(2.90	)

Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Per Share

Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:

	Three Months Ended March 31,
	2018		2017
Convertible preferred stock		31,750,297		25,280,436
Options to purchase common stock		4,433,459		2,332,221
Early exercised common stock subject to future vesting		1,058,270		1,199,098
Restricted stock accounted for as options		625,084		—
Warrants to purchase convertible preferred stock		763,501		763,501
Warrants to purchase common stock		96,610		96,610
Total		38,727,221		29,671,866

Organization - Additional Information (Details) $ / shares in Units, $ in Millions		3 Months Ended
	May 07, 2018 USD ($) $ / shares shares	Mar. 31, 2018 Segment $ / shares shares	Feb. 28, 2018 shares	Dec. 31, 2017 $ / shares shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of segments \| Segment		1
Common stock, shares authorized		140,000,000	122,000,000	122,000,000
Common stock, par value \| $ / shares		$ 0.0001		$ 0.0001
Preferred stock, shares authorized		103,283,818	91,739,149
IPO \| Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of shares issued	5,000,000
Net proceeds from issuance initial public offering after Deducting Underwriting Discounts Commissions and offering related transaction costs \| $	$ 76.1
Approximate underwriting discounts commissions and offering related transaction costs \| $	$ 8.9
Offering price to public to common stock \| $ / shares	$ 17.00
Outstanding shares of convertible preferred stock	32,073,149
Common stock, shares authorized	300,000,000
Common stock, par value \| $ / shares	$ 0.0001
Preferred stock, shares authorized	10,000,000
Preferred stock, par value \| $ / shares	$ 0.0001

Summary of Significant Accounting Policies - Additional Information (Details)		3 Months Ended		12 Months Ended
	Apr. 20, 2018	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit		$ (103,013,000)		$ (86,880,000)
Net losses		(16,133,000)	$ (8,934,000)
Net cash used in operating activities		(11,883,000)	$ (7,860,000)
Deferred offering costs		$ 1,707,000		$ 0
Off-balance sheet concentrations of credit risk description		As of March 31, 2018, the Company had no off-balance sheet concentrations of credit risk.
Off-balance sheet concentrations of credit risk		$ 0
Federal statutory income tax rate		21.00%		35.00%
Limitation of net operating loss deduction as percentage of current year taxable income		80.00%
Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Reverse split of shares of common and convertible preferred stock, conversion ratio	0.33898
Reverse split of shares of common and convertible preferred stock, effective date	Apr. 20, 2018
Amended and Restated Certificate of Incorporation \| Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Reverse split of shares of common and convertible preferred stock	1-for-2.95 reverse split

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 62,754	$ 7,298
Restricted cash	550	550
Total cash, cash equivalents, and restricted cash	$ 63,304	$ 7,848	$ 30,354	$ 89,736

Fair Value Measurements - Summary of Financial Assets Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	$ 71,281	$ 79,212
Long-term marketable securities		5,118
Total marketable securities		84,330
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	62,085	5,709
Short-term marketable securities	71,281
Total marketable securities		84,330
Assets fair value disclosure	133,366	90,039
Fair Value, Measurements, Recurring \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	62,085	5,709
Assets fair value disclosure	62,085	5,709
Fair Value, Measurements, Recurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	71,281
Total marketable securities		84,330
Assets fair value disclosure	71,281	84,330
Money market funds \| Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	62,085	5,709
Money market funds \| Fair Value, Measurements, Recurring \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	62,085	5,709
Commercial Paper \| Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	5,428	6,359
Commercial Paper \| Fair Value, Measurements, Recurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	5,428	6,359
Corporate Debt Securities \| Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	10,140	16,149
Corporate Debt Securities \| Fair Value, Measurements, Recurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	10,140	16,149
Asset-backed Securities \| Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	14,404	14,588
Long-term marketable securities		2,742
Asset-backed Securities \| Fair Value, Measurements, Recurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	14,404	14,588
Long-term marketable securities		2,742
US Government Debt Securities \| Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	41,309	42,116
Long-term marketable securities		2,376
US Government Debt Securities \| Fair Value, Measurements, Recurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	$ 41,309	42,116
Long-term marketable securities		$ 2,376

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair value assets hierarchy levels transfers amount	$ 0	$ 0

Marketable Securities - Summary of Marketable Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		$ 84,434
Unrealized Gains		1
Unrealized Losses		(105)
Fair Value		84,330
Short-Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 71,419	79,307
Unrealized Losses	(138)	(95)
Fair Value	71,281	79,212
Short-Term Marketable Securities \| Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	5,445	6,369
Unrealized Losses	(17)	(10)
Fair Value	5,428	6,359
Short-Term Marketable Securities \| Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	10,167	16,162
Unrealized Losses	(27)	(13)
Fair Value	10,140	16,149
Short-Term Marketable Securities \| Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	14,446	14,604
Unrealized Losses	(42)	(16)
Fair Value	14,404	14,588
Short-Term Marketable Securities \| US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	41,361	42,172
Unrealized Losses	(52)	(56)
Fair Value	$ 41,309	42,116
Long-Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		5,127
Unrealized Gains		1
Unrealized Losses		(10)
Fair Value		5,118
Long-Term Marketable Securities \| Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		2,752
Unrealized Losses		(10)
Fair Value		2,742
Long-Term Marketable Securities \| US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		2,375
Unrealized Gains		1
Fair Value		$ 2,376

Marketable Securities - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Investments Debt And Equity Securities [Abstract]
Available-for sale securities maximum remaining contractual maturity	2 years
Realized gains or losses on available-for-sale securities	$ 0

License Agreements - Additional Information (Details)	1 Months Ended	3 Months Ended
License Agreements - Additional Information (Details)	Feb. 29, 2016 USD ($) shares	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
Research And Development Arrangement Contract To Perform For Others [Line Items]
Funding provided for the research and development under the agreement in period	2020-02
Clinical Stage Biopharmaceutical Company \| Compound Library and Option Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Number of ordinary shares \| shares	275,766
Aggregate purchase price	$ 500,000
Commitment to invest an additional amount in future	500,000
Clinical Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Number of milestones achieved		0	0
Clinical Stage Biopharmaceutical Company \| Licensed Products
Research And Development Arrangement Contract To Perform For Others [Line Items]
Royalties due from sales		$ 0	$ 0
Maximum \| Clinical Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Funding provided for the research and development under the agreement	2,000,000
Licensing Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payments	$ 70,300,000
Licensing Agreement \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent consideration additional common stock issued \| shares	666,670
Licensing Agreements with Research Institutions
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone or royalty payments		$ 0

Commitments and Contingencies - Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) Lease	Mar. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Number of non-cancelable operating leases \| Lease	1
Operating leases, expiration date	Oct. 31, 2022
Rent expense, operating lease \| $	$ 0.5	$ 0.5

Commitments and Contingencies - Summary of Future Minimum Lease Payments under Non-cancellable Operating Leases (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018 (remaining 9 months)	$ 1,473
2019	2,012
2020	2,072
2021	2,135
2022	1,621
Total future minimum lease payments	$ 9,313

Related-Party Transactions - Additional Information (Details) $ in Thousands	1 Months Ended						3 Months Ended
	Apr. 30, 2018 USD ($)	Jan. 31, 2018 USD ($) shares	Oct. 31, 2017 USD ($) Promissory shares	Jul. 31, 2016 USD ($) shares	Apr. 30, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Mar. 31, 2018 USD ($) PromissoryNote Executive	Mar. 31, 2017 USD ($)	Apr. 04, 2018 USD ($)
Related Party Transaction [Line Items]
Number of promissory notes \| PromissoryNote							3
Number of executive officers \| Executive							2
Proceeds from issuance of convertible preferred stock							$ 54,951	$ 7,984
Executive Officer
Related Party Transaction [Line Items]
Due from related parties			$ 2,100
Restricted stock purchased by related party, shares \| shares			625,084
Stockholders \| Financing Activities
Related Party Transaction [Line Items]
Proceeds from issuance of convertible preferred stock							$ 3,000
Promissory Note \| Executive Officer
Related Party Transaction [Line Items]
Number of promissory notes \| Promissory			2
Due from related parties		$ 400		$ 200	$ 200	$ 200
Interest rate		2.50%		2.50%	2.50%	2.50%
Promissory Note \| Common Stock \| Executive Officer
Related Party Transaction [Line Items]
Early exercise options in aggregate \| shares		114,406		667,253	667,253	667,253
Full-recourse Notes \| Executive Officer \| Subsequent Event
Related Party Transaction [Line Items]
Due from related parties									$ 200
Promissory Note One \| Executive Officer
Related Party Transaction [Line Items]
Due from related parties			$ 1,600
Interest rate			1.85%
Promissory Note Two \| Executive Officer
Related Party Transaction [Line Items]
Due from related parties			$ 500
Interest rate			1.85%
Non-Recourse Promissory Note \| Executive Officer \| Subsequent Event
Related Party Transaction [Line Items]
Due from related parties									$ 500
Outstanding principal and accrued interest, forgiveness	$ 1,600

Convertible Preferred Stock and Common Stock - Additional Information (Details)		1 Months Ended		3 Months Ended
	Apr. 20, 2018	Apr. 30, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2017 USD ($)	May 07, 2018 $ / shares shares	Feb. 28, 2018 shares	Dec. 31, 2017 shares
Class Of Stock [Line Items]
Common stock, shares authorized			140,000,000	140,000,000			122,000,000	122,000,000
Preferred stock, shares authorized			103,283,818	103,283,818			91,739,149
Number of stock, shares issued			31,750,297	31,750,297				28,159,724
Net proceeds from sale of convertible preferred stock \| $				$ 54,951,000	$ 7,984,000
Subsequent Event
Class Of Stock [Line Items]
Conversion ratio of preferred stock	0.33898
Subsequent Event \| IPO
Class Of Stock [Line Items]
Common stock, shares authorized						300,000,000
Preferred stock, shares authorized						10,000,000
Number of preferred stock issued upon conversion						32,073,149
Share price \| $ / shares						$ 17.00
Common Stock
Class Of Stock [Line Items]
Conversion ratio of preferred stock				1
Common Stock \| IPO
Class Of Stock [Line Items]
Share price \| $ / shares			$ 15.3317	$ 15.3317
Percentage of preferred stock outstanding				60.00%
Common Stock \| Minimum \| IPO
Class Of Stock [Line Items]
Gross proceeds from public offering \| $				$ 30,000,000
Series C Convertible Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares authorized			11,544,669	11,544,669
Number of stock, shares issued			3,590,573	3,590,573
Shares price per share \| $ / shares			$ 15.3317	$ 15.3317
Net proceeds from sale of convertible preferred stock \| $			$ 54,900,000
Liquidation preference per share \| $ / shares			$ 15.3317	$ 15.3317
Series C Convertible Preferred Stock \| Subsequent Event
Class Of Stock [Line Items]
Number of stock, shares issued		322,852
Shares price per share \| $ / shares		$ 15.3317
Net proceeds from sale of convertible preferred stock \| $		$ 5,000,000
Series C Convertible Preferred Stock \| U.S. National Securities Exchange \| Minimum
Class Of Stock [Line Items]
Gross proceeds from public offering \| $			$ 30,000,000
Series C Convertible Preferred Stock \| Common Stock
Class Of Stock [Line Items]
Number of preferred stock issued upon conversion			1	1
Conversion price of preferred stock \| $ / shares			$ 15.3317	$ 15.3317
Series A-1 Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued			2,887,086	2,887,086				2,887,086
Conversion price of preferred stock \| $ / shares			$ 0.864	$ 0.864
Liquidation preference per share \| $ / shares			$ 0.864	$ 0.864
Series A-2 Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued			10,498,269	10,498,269				10,498,269
Conversion price of preferred stock \| $ / shares			$ 0.876	$ 0.876
Liquidation preference per share \| $ / shares			$ 0.876	$ 0.876
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued			14,774,369	14,774,369				14,774,369
Conversion price of preferred stock \| $ / shares			$ 12.125	$ 12.125
Liquidation preference per share \| $ / shares			$ 12.125	$ 12.125
Noncumulative Preferred Stock
Class Of Stock [Line Items]
Dividends rate of respective series of convertible preferred stock issuance price				6.00%

Convertible Preferred Stock and Common Stock - Summary of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	103,283,818	91,739,149
Convertible preferred stock, Shares Issued	31,750,297	28,159,724
Convertible preferred stock, Shares Outstanding	31,750,297	28,159,724
Convertible preferred stock, Liquidation Preference	$ 245,825	$ 190,825
Convertible preferred stock, Carrying Value	$ 228,907	$ 173,956
Series A-1
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	9,085,738	9,085,738
Convertible preferred stock, Shares Issued	2,887,086	2,887,086
Convertible preferred stock, Shares Outstanding	2,887,086	2,887,086
Convertible preferred stock, Liquidation Preference	$ 2,495	$ 2,495
Convertible preferred stock, Carrying Value	$ 2,457	$ 2,457
Series A-2
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	32,653,411	32,653,411
Convertible preferred stock, Shares Issued	10,498,269	10,498,269
Convertible preferred stock, Shares Outstanding	10,498,269	10,498,269
Convertible preferred stock, Liquidation Preference	$ 9,198	$ 9,198
Convertible preferred stock, Carrying Value	$ 9,214	$ 9,214
Series B
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	50,000,000	50,000,000
Convertible preferred stock, Shares Issued	14,774,369	14,774,369
Convertible preferred stock, Shares Outstanding	14,774,369	14,774,369
Convertible preferred stock, Liquidation Preference	$ 179,132	$ 179,132
Convertible preferred stock, Carrying Value	$ 162,285	$ 162,285
Series C
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	11,544,669
Convertible preferred stock, Shares Issued	3,590,573
Convertible preferred stock, Shares Outstanding	3,590,573
Convertible preferred stock, Liquidation Preference	$ 55,000
Convertible preferred stock, Carrying Value	$ 54,951

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		4 Months Ended
	Mar. 31, 2018	Jun. 30, 2013	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Performance Contingent Stock Options Granted to Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of stock options granted			400,000
Compensation cost recognized			$ 0
Performance Contingent Stock Options Granted to Employees \| Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable	53,575		53,575
Weighted average exercise price	$ 3.42		$ 3.42
Performance Contingent Stock Options Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation cost recognized			$ 0	$ 0
Stock option awards outstanding	329,498		329,498	275,922
Stock option awards, fair value	$ 700,000		$ 700,000	$ 300,000
Performance and Market Contingent Stock Options Granted to Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation cost recognized			$ 0	$ 0
Stock option awards outstanding	454,584		454,584	360,594
Stock option awards, fair value	$ 1,000,000		$ 1,000,000	$ 400,000
Performance and Market Contingent Stock Options Granted to Employees \| Vesting 1
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable	53,575		53,575
Vesting rights			(i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum price per share, (ii) a change in control with aggregate proceeds payable to the Company’s common stock at a minimum price per share, or (iii) an initial public offering that becomes effective at a minimum specified price per share
Trailing period for Initial public offering that becomes effective or achievement with a minimum market capitalization			30 days
Performance and Market Contingent Stock Options Granted to Employees \| Vesting 2
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable	107,152		107,152
Vesting rights			(i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum pre-money valuation, (ii) a change in control with a minimum aggregate proceeds payable to the Company’s common stock, or (iii) an initial public offering that becomes effective or an achievement of a minimum market capitalization, as measured by a trailing 30 day volume-weighted average price.
Performance and Market Contingent Stock Options Granted to Employees \| Executive Team
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average exercise price	$ 3.42		$ 3.42
Estimated fair value of stock options granted			700,000
Performance and Market Contingent Stock Options Granted to Employees \| Executive Team \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable	160,727		160,727
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	536,242		536,242
2018 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	4,289,936		4,289,936
Vesting period, options	4 years
2018 Incentive Award Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period from the date of grant, options	10 years
2018 Incentive Award Plan \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price as a percentage of estimated fair value of the shares on the date of grant	100.00%
2013 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period, options		4 years
Number of shares of common stock authorized for issuance	450,233		450,233
Exercise price					$ 3.42
Exercise price range, lower range limit					3.95
Exercise price range, upper range limit					$ 5.43
Estimated fair value of stock options granted			493,776
Stock option awards outstanding	4,263,969		4,263,969		4,196,203
2013 Equity Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period from the date of grant, options		10 years
2013 Equity Incentive Plan \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price as a percentage of estimated fair value of the shares on the date of grant		100.00%
Exercise price as a percentage of estimated grant date fair value of shares for a 10% shareholder		110.00%

Stock-Based Compensation - Summary of Stock Option Activity (Details) - 2013 Plan	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Number of Shares
Balances at December 31, 2017 \| shares	4,196,203
Granted \| shares	493,776
Exercised \| shares	(400,587)
Canceled \| shares	(25,423)
Balances at March 31, 2018 \| shares	4,263,969
Weighted-Average Exercise Price
Balances at December 31, 2017 \| $ / shares	$ 3.06
Granted \| $ / shares	5.79
Exercised \| $ / shares	3.10
Canceled \| $ / shares	3.39
Balances at March 31, 2018 \| $ / shares	$ 3.37

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested at December 31, 2017 \| shares	478,971
Shares, Vested \| shares	(119,742)
Shares, Unvested at March 31, 2018 \| shares	359,229
Weighted Average Grant Date Fair Value, Unvested at December 31, 2017	$ 4.57
Weighted Average Grant Date Fair Value, Granted	4.57
Weighted Average Grant Date Fair Value, Vested	4.57
Weighted Average Grant Date Fair Value, Unvested at March 31, 2018	$ 4.57

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,370	$ 413
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	974	170
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 396	$ 243

Net Loss per Common Share - Schedule of Reconciliation of Numerators and Denominators Used In Computing Net Loss From Continuing Operations Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net loss	$ (16,133)	$ (8,934)
Denominator:
Weighted average number of shares used in computing net loss per share, basic and diluted	3,437,345	3,079,551
Net loss per share, basic and diluted	$ (4.69)	$ (2.90)

Net Loss per Common Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	38,727,221	29,671,866
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	31,750,297	25,280,436
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	4,433,459	2,332,221
Early Exercised Common Stock Subject to Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	1,058,270	1,199,098
Restricted Stock Accounted For as Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	625,084
Warrants to Purchase Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	763,501	763,501
Warrants to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	96,610	96,610

Net Loss per Common Share - Additional Information (Details)	3 Months Ended
	Mar. 31, 2018 shares
Maximum
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares contingently issued	739,551