Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Stock-based Compensation Expense for Employees and Non-employees

In the accompanying consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows:

	Year Ended December 31,
	2016		2015		2014
	(in thousands)
Research and development	$	3,635	$	1,846	$	65
General and administrative		1,875		1,340		237
Total stock-based compensation	$	5,510	$	3,186	$	302

Schedule of Estimated Grant-date Fair Value of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions

The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions:

	Year Ended December 31,
	2016			2015			2014
Expected term (in years)		5.1 – 9.9			5.2 – 10.0			5.0 – 6.0
Expected volatility		77 – 87	%		75 – 84	%		81 – 83	%
Risk-free interest rate		1.1 – 2.4	%		1.5 – 2.4	%		1.5 – 1.8	%
Expected dividend rate		—	%		—	%		—	%

Summary of Stock-Based Compensation Activity

A summary of activity under the 2008 Plan and 2015 Plan and related information is as follows:

				Options Outstanding
	Shares Available for Grant			Number of Shares Outstanding			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value of Outstanding Options (in thousands)
Outstanding—December 31, 2013		637,768			582,152		$	0.37		8.63	$	48
Awards authorized		3,221,477
Options granted		(2,309,681	)		2,309,681			0.93
Options exercised		—			(752,698	)		0.54
Options cancelled		537			(1,039	)		2.08
Outstanding—December 31, 2014		1,550,101			2,138,096			0.91		9.29		3,153
Awards authorized		3,400,000
Options granted		(1,471,664	)		1,471,664			8.06
Options exercised		—			(41,505	)		0.76
Options cancelled		45,442			(45,442	)		7.76
Outstanding—December 31, 2015		3,523,879			3,522,813			3.81		8.78		40,425
Awards authorized		1,202,324
Options granted		(3,710,980	)		3,710,980			3.22
Options exercised		—			(312,841	)		0.67
Options cancelled		377,298			(377,298	)		7.04
Outstanding—December 31, 2016		1,392,521			6,543,654		$	3.44		8.82	$	888
Exercisable—December 31, 2016					2,614,587		$	2.41		7.99	$	888
Vested and expected to vest—December 31, 2016					6,352,640		$	3.43		8.81	$	877