Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2014

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

Summary of Option Activity

The following is a summary of option activity under the 2005 Plan and the 1995 Plan for the six months ended June 30, 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		5,410,000		$	5.59		—		$	—
Exercised		(493,462	)	$	1.87		(32,450	)	$	4.81
Canceled		(225,775	)	$	3.11		(137,000	)	$	5.97
Outstanding at June 30, 2014		16,478,863		$	3.07		18,700		$	2.51
Shares exercisable at June 30, 2014		6,000,837		$	1.89		18,700		$	2.51
Shares available for grant at June 30, 2014		5,416,744

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

	Three Months Ended June 30,				Six Months Ended  June 30,
	2014		2013		2014		2013
Weighted-average Black-Scholes fair value of stock options granted	$	1.92	$	1.01	$	2.48	$	1.00
Risk-free interest rate		1.33%-1.39%		0.54%-0.96%		1.24%-2.22%		0.54%-1.34%
Dividend yield		0%		0%		0%		0%
Volatility		52.87%-53.81%		55.81%-66.02%		52.47%-67.93%		55.81%-73.72%
Expected term (in years)		4.10-4.26		3.98-4.25		4.04-6.96		3.98-7.05
Expected forfeiture rate		0%-23.15%		0%-23.15%		0%-23.15%		0%-23.15%

Stock-Based Compensation Expense

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2013		2014		2013
Research and development	$	651	$	296	$	1,175	$	513
General and administrative		1,201		342		1,717		608
Total stock-based     compensation expense	$	1,852	$	638	$	2,892	$	1,121

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the six months ended June 30, 2014:

	Number of Shares		Per Share Weighted- Average Grant-Date Fair Value
Outstanding and Unvested at January 1, 2014		16,667	$	1.39
Restricted stock granted		—	$	—
Restricted stock vested		—	$	—
Restricted stock forfeited		—	$	—
Outstanding and Unvested at June 30, 2014		16,667	$	1.39

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 June 30, 2014		Six Months Ended June 30, 2014
Range of Black-Scholes fair values of ESPP shares granted		$0.97-$1.79		$0.97-$1.79
Risk-free interest rate		0.11%-0.14%		0.11%-0.14%
Dividend yield		0%		0%
Volatility		53.80%-67.57%		53.80%-67.57%
Expected term (in years)		0.5-1.0		0.5-1.0
Expected forfeiture rate		5%		5%