15. Stock-Based Compensation

In 2008, the Company adopted the 2008 Stock Incentive Plan (as amended, the “2008 Plan”) for employees, officers, directors, consultants and advisors. The 2011 Stock Incentive Plan (the “2011 Plan”) became effective upon closing of the Company’s initial public offering in April 2012. Upon effectiveness of the 2011 Plan, no further awards were available to be issued under the 2008 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. Additional shares also become available for grant by reason of the forfeiture, cancellation, expiration or termination of existing awards. The Company registered 3.4 million, 3.6 million and 3.7 million of additional shares of common stock related to the 2011 Plan in February 2013, March 2014 and February 2015, respectively. As of December 31, 2015, there were 2.5 million shares remaining available for grant under the 2011 Plan.

During the years ended December 31, 2015, 2014 and 2013, the Company issued options to purchase 3.7 million, 3.9 million and 3.3 million shares of common stock, respectively. These options generally vest over a three-year period for employees. Options granted to directors during the period from April 2012 through December 2013 vest over a one-year period. All other options granted to directors vest immediately. During the year ended December 31, 2013, the Company also issued options to purchase less than 0.1 million shares of common stock to non-employees. The assumptions used to estimate the fair value of options granted to non-employees at the date of grant were materially consistent with those used for employee and director grants. The Company did not grant any options to purchase common stock to non-employees during the years ended December 31, 2015 or 2014.

The Company recognized stock-based compensation expense as follows:


	Years ended December 31,
(in thousands)	2015		2014		2013
Employee awards:
Research and development	$	8,271	$	6,864	$	5,954
General and administrative		7,022		6,065		4,808

Stock-based compensation for employee awards		15,293		12,929		10,762
Stock-based compensation for non-employee awards		58		268		(29	)

Total stock-based compensation	$	15,351	$	13,197	$	10,733

The fair value of stock options granted to employees during the years ended December 31, 2015, 2014 and 2013 was estimated at the date of grant using the following assumptions:


	Years ended December 31,
	2015	2014	2013
Risk-free interest rate	1.5 - 1.8%	1.6 - 2.0%	0.1 - 1.9%
Expected dividend yield	0%	0%	0%
Expected term	5.0 - 5.9 years	5.0 - 5.9 years	5.3 - 5.9 years
Expected volatility	66 - 67%	64 - 72%	67 - 70%

The Company uses the simplified method to calculate the expected term, as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term. The computation of expected volatility is based on the historical volatility of comparable companies from a representative peer group selected based on industry and market capitalization. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. Management estimates expected forfeitures based on historical experience and recognizes compensation costs only for those equity awards expected to vest.

The following table summarizes stock option activity during the year ended December 31, 2015:


(in thousands, except per share amounts)	Shares			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2014		19,556		$	4.47		6.17	$	133,599
Granted		3,720		$	9.63
Exercised		(3,606	)	$	2.75
Forfeited		(459	)	$	7.36

Outstanding at December 31, 2015		19,211		$	5.72		6.24	$	47,963
Vested and expected to vest at December 31, 2015		18,923		$	5.68		6.20	$	47,818
Exercisable at December 31, 2015		14,783		$	5.01		5.48	$	44,173

The weighted-average grant date fair value of stock options granted during the years ended December 31, 2015, 2014 and 2013 was $5.80, $3.41 and $3.59, respectively.

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. The aggregate intrinsic value of options exercised during the years ended December 31, 2015, 2014 and 2013 was $30.9 million, $19.8 million and $2.7 million, respectively.

As of December 31, 2015, there was $18.3 million of total unrecognized compensation cost related to unvested employee stock awards. As of December 31, 2015, the Company expects to recognize those costs over a weighted average period of approximately 1.8 years.