Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2015

Equity [Abstract]

Schedule of Shares of Common Stock Reserved for Future Issuance

Shares of common stock reserved for future issuance were as follows:

	September 30, 2015	December 31, 2014
	(unaudited)
Shares to be issued under the employee stock purchase plan	321,309	155,220
Shares to be issued upon exercise of outstanding stock options	2,307,967	1,907,091
Shares available for future stock option grants	1,334,553	1,274,067
Shares of common stock reserved for future issuance	3,963,829	3,336,378

Summary of Stock Options

Summary stock option information is as follows:

	OPTIONS OUTSTANDING		WEIGHTED- AVERAGE EXERCISE PRICE		WEIGHTED- AVERAGE REMAINING CONTRACT TERM (in years)	AGGREGATE INTRINSIC  VALUE (in thousands)
Outstanding at December 31, 2014	1,907,091		$	4.16	8.16	$	50,847
Granted (unaudited)	711,394		$	27.50
Exercised (unaudited)	(213,790	)	$	1.39
Forfeited (unaudited)	(96,728	)	$	18.61
Outstanding at September 30, 2015 (unaudited)	2,307,967		$	10.97	8.11	$	14,549
Vested and expected to vest after September 30, 2015 (unaudited)	2,177,111		$	10.59	8.06	$	14,094
Exercisable as of September 30, 2015 (unaudited)	958,335		$	2.80	7.06	$	9,274

Schedule of Stock-Based Compensation Expense Recognized

Total stock-based compensation expense recognized in our condensed statement of operations is as follows (in thousands):

	Three Months Ended September 30,					Nine Months Ended September 30,
	2015			2014		2015		2014
	(unaudited)
Employee:
Research and development	$	480		$	131	$	1,369	$	274
General and administrative	866			220		2,811		449
Non-Employee:
Research and development	(8		)	73		88		155
General and administrative	69			—		183		—
Total stock-based compensation expense	$	1,407		$	424	$	4,451	$	878

Schedule of Fair Value of Stock Options Granted to Employees

The fair values of stock options granted to employees were calculated using the following assumptions:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2015	2014	2015	2014
	(unaudited)
Weighted-average estimated fair value	$12.60	$12.05	$21.20	$6.91
Risk-free interest rate	1.64% - 1.86%	1.96% - 2.05%	1.50% - 1.86%	1.76% - 2.16%
Expected term of options (in years)	6.08	5.84 - 6.11	5.50 - 6.08	5.50 - 6.75
Expected stock price volatility	78% - 83%	90% - 93%	78% - 91%	90% - 93%
Expected dividend yield	—	—	—	—