Stock-Based Compensation

3 Months Ended

Mar. 31, 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 three months ended March 31, 2012 the Company did not issue options to purchase shares of common stock. During the three months ended March 31, 2013, the Company issued options to purchase 2.7 million shares of common stock. At March 31, 2013, there were 1.9 million shares remaining available for grant under the 2011 Plan.

The weighted-average assumptions used to estimate the fair value of options granted to employees at the date of grant for the three months ended 2013 were as follows:

	Three months ended March 31, 2013
Risk-free interest rate	1.1%
Expected dividend yield	0%
Expected term	5.8-5.9 years
Expected volatility	67%

These options generally vest over a three-year period for employees. The Company recognized stock-based compensation expense as follows for the three months ended March 31, 2012 and 2013:

	Three months ended March 31,
(in thousands)	2012		2013
Employee awards:
Research and development	$	909	$	1,284
General and administrative	430		1,090
Stock-based compensation for employee awards	1,339		2,374
Stock-based compensation for non-employee awards	—		12
Total stock-based compensation	$	1,339	$	2,386

The following table summarizes stock option activity during the three months ended March 31, 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	2,719		$	6.35
Exercised	(90	)	$	2.18
Forfeited	(31	)	$	6.76
Outstanding, March 31, 2013	20,664		$	3.87	6.71	$	51,276
Exercisable, March 31, 2013	14,177		$	2.66	5.52	$	49,725
Vested and expected to vest, March 31, 2013	20,154		$	3.80	6.64	$	51,249

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.