Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2014

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

The following is a summary of option activity under the 2005 Plan and the 1995 Plan for the year ended December 31, 2014:

	2005 Stock Incentive Plan			1995 Stock Option Plan
	Stock Options	Weighted-Average Exercise Price		Stock Options		Weighted-Average Exercise Price
Outstanding at January 1, 2014	11,788,100	$	1.87		188,150	$	5.04
Granted	6,418,000	$	5.50		—	$	—
Exercised	(898,302)	$	1.67		(32,450)	$	4.81
Forfeited	(378,450)	$	2.79		—	$	—
Expired	(1,250)	$	1.57		(120,700)	$	5.76
Outstanding at December 31, 2014	16,928,098	$	3.24		35,000	$	2.21
Vested and expected to vest at December 31, 2014	15,445,657	$	3.09		35,000	$	2.21
Shares exercisable at December 31, 2014	6,069,997	$	2.07		35,000	$	2.21

Shares available for grant at December 31, 2014	4,531,369

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

The fair value of the stock options granted for the years ended December 31, 2014, 2013 and 2012, was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:

	2014		2013		2012
Weighted average fair value of options granted		$2.39		$1.07		$0.71
Risk-free interest rate		1.24%-2.22%		0.54%-1.36%		0.55%-1.54%
Dividend yield		0%		0%		0%
Volatility		52.47%-67.93%		51.55%-73.72%		75.5%-78.6%
Expected term (in years)		4.04-6.96%		3.91-7.05%		3.34-7.09%
Expected forfeiture rate		0%-23.15%		0%-23.15%		0%-23.15%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the year ended December 31, 2014:

			Per Share Weighted-Average
	Number of Shares		Grant-Date Fair Value
Outstanding at January 1, 2014		16,667	$	1.39
Restricted stock granted		15,000	$	4.48
Restricted stock vested		(16,667)	$	1.39
Restricted stock forfeited		—	$	—
Outstanding at December 31, 2014		15,000	$	4.48

Stock-Based Compensation Expense

The Company recorded stock-based compensation expense for awards issued under the above mentioned plans in the statements of operations as follows (in thousands):

	Years ended December 31,
	2014		2013		2012
Research and development	$	2,843	$	1,262	$	873
General and administrative		3,247		1,218		1,218
Total stock-based compensation expense	$	6,090	$	2,480	$	2,091

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:

	Year Ended	Year Ended
	December 31,	December 31,
	2014	2013
Range of Black-Scholes fair value of ESPP shares granted	$0.78-$2.08		$0.78
Risk-free interest rate	0.04%-0.24%		0.04%
Dividend yield	0%		0%
Volatility	50.80%-67.57%		50.80%
Expected term (in years)	0.5-1.5		0.5
Expected forfeiture rate	5%		5%