Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Stock-Based Compensation Expense by Award Type

The Company’s stock-based compensation expense by award type is as follows (in thousands):

	Years Ended December 31,
	2016		2015		2014
Stock options	$	6,950	$	2,921	$	319
Restricted stock units		23		—		—
Employee stock purchase plan		58		—		—
	$	7,031	$	2,921	$	319

Stock-Based Compensation Expense

The Company has recorded aggregate stock-based compensation expense in the statement of operations and comprehensive loss as follows (in thousands):

	Years Ended December 31,
	2016		2015		2014
Research and development	$	2,743	$	1,760	$	60
General and administrative		4,288		1,161		259
	$	7,031	$	2,921	$	319

Schedule of Fair Value of Options Granted to Employees and Directors Valuation Assumptions

The fair value of options granted to employees and directors was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:

	Years Ended December 31,
	2016			2015			2014
Expected volatility		75	%		68	%		64	%
Expected term (in years)		6.2			6.1			6.0
Risk-free interest rate		1.5	%		1.7	%		2.0	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Summary of Unvested RSUs Activity Under 2015 Plan

The following table summarize restricted stock unit activity under the 2015 Plan (in thousands, except per share data):

			Weighted-
			average
			Grant Date
	Shares		Fair Value
Unvested balance at December 31, 2015		—	$	—
Granted		40	$	20.90
Vested		—	$	—
Forfeited		—	$	—
Unvested balance at December 31, 2016		40	$	20.90

Non Employee Stock Option [Member]

Schedule of Non-Employees Stock Purchase Plan Valuation Assumptions

The fair value of options granted to non-employees was estimated at each measurement date using the Black-Scholes valuation model with the following weighted-average assumptions:

	Years Ended December 31,
	2016			2015			2014
Expected volatility		83	%		75	%		65	%
Expected term (in years)		8.5			9.5			9.9
Risk-free interest rate		1.7	%		2.1	%		2.4	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

2014 and 2015 Equity Incentive Plan [Member]

Summary of Stock Option Activity

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2015		3,684		$	5.52		9.1	$	44,472
Granted		1,796		$	12.58
Exercised		(163	)	$	1.10
Cancelled or forfeited		(199	)	$	15.04
Outstanding at December 31, 2016		5,118		$	7.77		8.5	$	57,400
Exercisable at December 31, 2016		2,147		$	3.90		8.0	$	32,198
Vested and expected to vest at December 31, 2016		5,118		$	7.77		8.5	$	57,400

(a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.