Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2014

Stock-Based Compensation Expense Related to Common Stock Options, Common Stock, Restricted Common Stock and Common Stock Warrants

Following is the stock-based compensation expense related to common stock options, common stock, restricted common stock and common stock warrants:

	Three Months Ended March 31,
	2014		2013
Research and development	$	638	$	319
General and administrative		1,010		533
Total stock-based compensation expense	$	1,648	$	852

Summary of Stock Option Activity

The following table summarizes the stock option activity in the Company’s equity incentive plans, including non-plan grants to Company executives, from December 31, 2013 through March 31, 2014:

	Shares			Weighted Average Exercise Price
Outstanding, December 31, 2013		3,348,705		$	4.70
Granted		326,500			13.38
Exercised		(288,749	)		2.07
Options forfeited/cancelled		(21,258	)		3.59
Outstanding, March 31, 2014		3,365,198		$	5.78

Weighted Average Assumptions Used to Determine Fair Value of Options Granted

The fair value of all other options granted is determined using the Black-Scholes option-pricing model. The following weighted average assumptions were used:

	Three Months Ended March 31,	Three Months Ended March 31,
	2014	2013
Risk-free interest rate	1.58%	0.80%
Expected life of the options	6.0 years	5.0 years
Expected volatility of the underlying stock	114%	117%
Expected dividend rate	0%	0%