16. 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 4.1 million, 3.7 million and 3.6 million of additional shares of common stock related to the 2011 Plan in February 2016, February 2015 and March 2014, respectively. As of December 31, 2016, there were 4.7 million shares remaining available for grant under the 2011 Plan.

During the years ended December 31, 2016, 2015 and 2014, the Company issued options to purchase 4.3 million, 3.7 million and 3.9 million shares of common stock, respectively. These options generally vest over a three-year period for employees. Options granted to directors vest immediately.

The fair value of stock options granted to employees during the years ended December 31, 2016, 2015 and 2014 was estimated at the date of grant using the following assumptions:

	Years Ended December 31,
	2016	2015	2014
Risk-free interest rate	1.1 – 2.0%	1.5 – 1.8%	1.6 – 2.0%
Expected dividend yield	0%	0%	0%
Expected term	5.0 – 5.8 years	5.0 – 5.9 years	5.0 – 5.9 years
Expected volatility	67 – 69%	66 – 67%	64 – 72%

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 the expected term of its stock options. Under this approach, the expected term is calculated to be the average of the ten-year contractual term of the option and the weighted-average vesting term of the option, taking into consideration multiple vesting tranches. 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 Company recognized stock-based compensation expense during the years ended December 31, 2016, 2015 and 2014 as follows:

	Years Ended December 31,
(in thousands)	2016			2015		2014
Employee awards:
Research and development expense	$	6,503		$	8,271	$	6,864
Selling, general and administrative expense		7,331			7,022		6,065
Restructuring expense		1,371			—		—
Stock-based compensation expense for employee awards		15,205			15,293		12,929
Stock-based compensation expense for non-employee awards		1			58		268
Less: stock-based compensation expense capitalized to inventory		(341	)		—		—
Total stock-based compensation expense	$	14,865		$	15,351	$	13,197

The following table summarizes stock option activity during the year ended December 31, 2016:

(in thousands, except per share amounts)	Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2015		19,211		$	5.72		6.24	$	47,963
Granted		4,258		$	5.57
Exercised		(1,958	)	$	3.29
Forfeited		(2,487	)	$	7.04
Outstanding at December 31, 2016		19,024		$	5.77		5.97	$	7,564
Vested and expected to vest at December 31, 2016		18,794		$	5.76		5.93	$	7,564
Exercisable at December 31, 2016		14,968		$	5.49		5.24	$	7,564

The weighted-average grant date fair value of stock options granted during the years ended December 31, 2016, 2015 and 2014 was $3.32, $5.80 and $3.41, 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, 2016, 2015 and 2014 was $5.6 million, $30.9 million and $19.8 million, respectively.

As of December 31, 2016, there was $12.7 million of total unrecognized stock-based compensation expense related to unvested employee stock options. The Company expects to recognize this expense over a weighted-average period of approximately 1.7 years.