Stock-Based Compensation Plans (Tables)

9 Months Ended

Sep. 30, 2015

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock option activity under the Company’s 2015 Plan and 2009 Plan was as follows:

				Options Outstanding
	Shares Available for Grant			Number of Options			Weighted- Average Exercise Price		Aggregate Intrinsic Value
									(In thousands)
Balance – December 31, 2014		3,154,755			5,970,382		$	0.80
Authorized		6,494,292			—
Granted		(4,308,197	)		4,308,197		$	7.23
Exercised		—			(293,250	)	$	0.61
Canceled		96,474			(96,474	)	$	1.74
Balance – September 30, 2015		5,437,324			9,888,855		$	3.60	$	161,161
Options exercisable – September 30, 2015					4,536,982		$	1.23	$	82,455
Options vested and expected to vest – September 30, 2015					9,400,157		$	3.53	$	153,714

Summary of Stock-Based Compensation

Total stock-based compensation expense recognized was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
Research and development	$	499	$	54	$	1,250	$	135
General and administrative		1,388		110		3,982		233
Total stock-based compensation expense	$	1,887	$	164	$	5,232	$	368

Schedule of Black-Scholes Option Pricing Model

The fair value of stock-based awards granted to employees during the nine months ended September 30, 2015 was estimated at the date of grant using the following assumptions:

	2015 Plan	2015 ESPP
Expected term (in years)	5.3 - 6.1	0.5
Volatility	70.2% - 82.8%	71.7%
Risk-free interest rate	1.38% - 1.93%	0.1%
Dividend yield	—%	—%