Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2015

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

Summary of Option Activity

Stock Options Awards

The following is a summary of option activity under the 2005 Plan and the 1995 Stock Option Plan (“1995 Plan”) for the three months ended March 31, 2015:

	2005 Plan				1995 Plan
	Stock Options		Weighted-Average Exercise Price		Stock Options		Weighted-Average Exercise Price
Outstanding at January 1, 2015		16,928,098	$	3.24		35,000	$	2.21
Granted		22,500	$	6.70		—	$	—
Exercised		(545,251)	$	2.22		(35,000)	$	2.21
Canceled		(121,375)	$	3.74		—	$	—
Outstanding at March 31, 2015		16,283,972	$	3.27		—	$	—
Shares exercisable at March 31, 2015		7,811,347	$	2.45		—	$	—

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 2005 Plan was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended March 31,
	2015		2014
Weighted-average Black-Scholes fair value of stock options granted	$	2.92	$	2.63
Risk-free interest rate		1.19%		1.24%-2.22%
Dividend yield		0%		0%
Volatility		53.58%-53.89%		52.47%-67.93%
Expected term (in years)		4.26		4.04-6.96
Expected forfeiture rate		16.33%		0%-23.15%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the three months ended March 31, 2015:

	Number of Shares		Per Share Weighted-Average Grant-Date Fair Value
Outstanding and Unvested at January 1, 2015		15,000	$	4.48
Restricted stock granted		—	$	—
Restricted stock vested		—	$	—
Restricted stock forfeited		—	$	—
Outstanding and Unvested at March 31, 2015		15,000	$	4.48

Stock-Based Compensation Expense

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

	Three Months Ended March 31,
	2015		2014
Research and development	$	1,032	$	524
General and administrative		900		516
Total stock-based compensation expense	$	1,932	$	1,040

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 March 31,
	2015		2014
Weighted-average Black-Scholes fair value of stock options granted		$1.06-$2.24		$0.97-$1.79
Risk-free interest rate		0.05%-0.35%		0.11%-0.14%
Dividend yield		0%		0%
Volatility		40.79%-64.24%		53.80%-67.57%
Expected term (in years)		0.5-2.0		0.5-1.0
Expected forfeiture rate		5%		5%