Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

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,
	2015		2014		2013
	(in thousands)
Research and development	$	1,846	$	65	$	5
General and administrative		1,340		237		17
Total stock-based compensation	$	3,186	$	302	$	22

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,
	2015			2014			2013
Expected term (in years)		5.2 – 10.0			5.0 – 6.0			5.0 – 6.0
Expected volatility		75 – 84	%		81 – 83	%		79 – 82	%
Risk-free interest rate		1.5 – 2.4	%		1.5 – 1.8	%		0.7 – 1.6	%
Expected dividend rate		—	%		—	%		—	%

Summary of Stock-Based Compensation Activity

A summary of activity under the 2005 Plan, 2008 Plan and 2015 Plan and related information is as follows:

				Options Outstanding									Non-Vested Stock
	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)		Number of Shares Outstanding			Weighted- Average Grant Date Fair Value Per Share
Outstanding—December 31, 2012		555,135			271,276		$	0.32		7.66	$	—		404		$	0.30
Awards authorized		520,134
Options granted		(403,759	)		403,759			0.37
Non-vested stock granted		(35,353	)		—									35,353			0.29
Options exercised		—			(91,272	)		0.27
Non-vested stock vested		—			—									(35,757	)		0.29
Options cancelled		1,611			(1,611	)		0.26
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		—			—
Exercisable—December 31, 2015					2,152,742		$	1.04		8.35	$	30,140
Vested and expected to vest—December 31, 2015					3,417,287		$	3.74		8.77	$	39,437