Stock-Based Compensation

6 Months Ended

Jun. 30, 2013

Stock-Based Compensation

Stock-Based Compensation

9. Stock-Based Compensation

As of December 31, 2012, there were 1.3 million shares of common stock available to be issued under the 2011 Stock Incentive Plan, (the “2011 Plan”). The 2011 Plan is administered by the Board of Directors of the Company and permits the Company to grant incentive and non-qualified stock options, stock appreciation rights, restricted stock, restricted stock units and other stock-based awards.

In January 2013, 3.4 million shares of common stock became available for grant to employees, officers, directors and consultants under the 2011 Plan. During the six months ended June 30, 2012 and 2013, the Company issued options to purchase 0.3 million and 3.0 million shares of common stock, respectively. At June 30, 2013, there were 1.7 million shares remaining available for grant under the 2011 Plan.

The range of assumptions used to estimate the fair value of options granted to employees and directors at the date of grant for the three and six months ended 2013 were as follows:

	Three months ended June 30,		Six months ended June 30,
	2012	2013	2012	2013
Risk-free interest rate	0.9-1.1%	0.1-1.4%	0.9-1.1%	0.1-1.4%
Expected dividend yield	0%	0%	0%	0%
Expected term	5.8-5.9 years	5.3-5.9 years	5.8-5.9 years	5.3-5.9 years
Expected volatility	71-72%	67-68%	71-72%	67-68%

These options generally vest over a three-year period for employees. The Company recognized stock-based compensation expense as follows for the three and six months ended June 30, 2012 and 2013:

	Three months ended June 30,				Six months ended June 30,
(in thousands)	2012		2013		2012		2013
Employee awards:
Research and development	$	945	$	1,651	$	1,853	$	2,935
General and administrative	432		1,307		862		2,397
Stock-based compensation for employee awards	1,377		2,958		2,715		5,332
Stock-based compensation for non-employee awards	33		72		34		84
Total stock-based compensation	$	1,410	$	3,030	$	2,749	$	5,416

The following table summarizes stock option activity during the six months ended June 30, 2013:

					Weighted
					Average
	Number		Weighted		Remaining	Aggregate
	of		Average		Contractual	Intrinsic
(in thousands, except per share amounts)	Shares		Exercise Price		Term	Value
Outstanding, December 31, 2012	18,066		$	3.50	6.54	$	51,486
Granted	3,076		$	6.24
Exercised	(499	)	$	2.41
Forfeited	(196	)	$	5.94
Outstanding, June 30, 2013	20,447		$	3.91	6.59	$	60,107
Exercisable, June 30, 2013	14,560		$	2.85	5.53	$	57,350
Vested and expected to vest, June 30, 2013	20,026		$	3.85	6.53	$	59,982

The aggregate intrinsic value was calculated as the difference between the exercise price of the stock options and the fair value of the underlying common stock as of the respective balance sheet date.