Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2015

Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]

The following is a summary of option activity under the 2015 Plan, 2005 Plan and the 1995 Stock Option Plan (“1995 Plan”) for the nine months ended September 30, 2015:

	2015 Plan				2005 Plan				1995 Plan
			Weighted-				Weighted-				Weighted-
			Average				Average				Average
	Stock		Exercise		Stock		Exercise		Stock		Exercise
	Options		Price		Options		Price		Options		Price
Outstanding at January 1, 2015		—	$	—		16,928,098	$	3.24		35,000	$	2.21
Granted		7,661,441	$	9.02		22,500	$	6.70		—	$	—
Exercised		—	$	—		(1,159,395)	$	2.30		(35,000)	$	2.21
Canceled		(102,188)	$	9.00		(191,250)	$	3.71		—	$	—
Outstanding at September 30, 2015		7,559,253	$	9.02		15,599,953	$	3.30		—	$	—
Shares exercisable at September 30, 2015		240,000	$	8.94		8,275,828	$	2.53		—	$	—
Shares available for grant at September 30, 2015		17,440,747

Assumptions used to Estimate Grant Date Fair Value of Stock Options granted using Black-Scholes Option-Pricing Model

The fair value of stock options granted under the 2015 Plan and 2005 Plan was estimated at the date of grant or the date upon which the 2015 Plan was approved by the Company’s stockholders for stock options granted prior to that time using the Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2015		2014		2015		2014
Weighted-average Black- Scholes fair value of stock options granted		$4.70		$1.92		$4.43	$2.41
Risk-free interest rate		1.32%-1.39%		1.43%-1.51%		1.19%-2.13%	1.24%-2.22%
Dividend yield		0%		0%		0%	0%
Volatility		54.93%-57.17%		52.75%-54.25%		53.58%-68.39%	52.47%-67.93%
Expected term (in years)		4.27-4.60		4.10-4.27		3.98-7.34	4.04-6.96
Expected forfeiture rate		14.18%-16.33%		14.18%-16.33%		0%-16.33%	0%-23.15%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the nine months ended September 30, 2015:

			Per Share
			Weighted-
			Average
	Number of		Grant-Date
	Shares		Fair Value
Outstanding and Unvested at January 1, 2015		15,000	$	4.48
Restricted stock granted		—	$	—
Restricted stock vested		(15,000)	$	4.48
Restricted stock forfeited		—	$	—
Outstanding and Unvested at September 30, 2015		—	$	—

Stock-Based Compensation Expense

The Company recorded all stock-based compensation expense in the consolidated statements of operations as follows (in thousands):

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2015		2014		2015		2014
Research and development	$	2,240	$	780	$	4,361	$	1,955
General and administrative		2,525		911		4,917		2,628
Total stock-based compensation expense	$	4,765	$	1,691	$	9,278	$	4,583

Employee Stock Purchase Plan [Member]

Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]

Assumptions used to Estimate Grant Date Fair Value of Stock Options granted using Black-Scholes Option-Pricing Model

The ESPP is considered compensatory for financial reporting purposes. As such, the fair value of ESPP shares was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2015		2014		2015		2014
Range of Black-Scholes fair values of ESPP shares granted		$1.20-$3.38		$0.97-$2.08		$1.06-$3.38		$0.78-$2.08
Risk-free interest rate		0.07%-0.35%		0.05%-0.24%		0.05%-0.35%		0.04%-0.24%
Dividend yield		0%		0%		0%		0%
Volatility		40.79%-64.24%		51.10%-67.57%		40.79%-64.24%		50.80%-67.57%
Expected term (in years)		0.5-2.0		0.5-1.5		0.5-2.0		0.5-1.5
Expected forfeiture rate		5%		5%		5%		5%