Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 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 three months ended March 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		4,009,000		$	6.03		-		$	-
Exercised		(379,287	)	$	2.00		(32,450	)	$	4.81
Canceled		(93,750	)	$	2.60		(137,000	)	$	5.97
Outstanding at March 31, 2014		15,324,063		$	2.95		18,700		$	2.51
Shares exercisable at March 31, 2014		5,586,387		$	1.87		18,700		$	2.51
Shares available for grant at March 31, 2014		2,685,719

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 March 31,
	2014	2013
Weighted-average Black-Scholes fair value of stock options granted	$2.63	$1.00
Risk-free interest rate	1.24%-2.22%	0.62%-1.34%
Dividend yield	0%	0%
Volatility	52.47%-67.93%	69.23%-73.72%
Expected term (in years)	4.04-6.96	4.06-7.05
Expected forfeiture rate	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 March 31,
	2014		2013
Research and development	$	524	$	217
General and administrative		516		266
Total stock-based compensation expense	$	1,040	$	483

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the three months ended March 31, 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 March 31, 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 March 31,
	2014
Weighted-average Black-Scholes fair value of ESPP shares granted	$0.97-$1.79
Risk-free interest rate	0.11%-0.14%
Dividend yield	0%
Volatility	53.80%-67.57%
Expected term (in years)	0.5-1.0
Expected forfeiture rate	5%