Stock-Based Compensation

6 Months Ended

Jun. 30, 2016

Stock-Based Compensation [Abstract]

Stock-Based Compensation

6. Stock-Based Compensation

 Under the provisions of the Company's 2013 Stock Incentive Plan (2013 Plan), the number of shares of common stock reserved for issuance will automatically increase on January 1 of each year from January 1, 2014 through and including January 1, 2023, by the lesser of (a) 1,960,168 shares, (b) 4.0 % of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or (c) the number of shares of common stock determined by the Board of Directors. During the six months ended June 30, 2016, the maximum number of shares of common stock authorized to be issued by the Company under the 2013 Plan was increased to 5,375,064.   As of June 30, 2016, there were options to purchase an aggregate of 2,587,788 shares of common stock outstanding at a weighted average exercise price of $26.32 per share under the 2013 Plan.

The following stock-based compensation amounts were recognized for the periods indicated (in thousands):

	Three Months Ended June 30,						Six Months Ended June 30,
	2016			2015			2016			2015
Research and development		$	1,445		$	881		$	2,841		$	1,691
General and administrative			1,678			1,021			3,283			1,842
Total stock-based compensation expense		$	3,123		$	1,902		$	6,124		$	3,533

Employee Stock Options

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:

	Six Months Ended June 30,
	2016	2015
Expected dividend yield	0%	0%
Expected volatility	63.7 % - 65.4 %	74 %
Risk-free interest rate	1.3% - 2.1%	1.6% - 2.1%
Expected term	6.25 years	6.25 years

The following table summarizes stock option and restricted stock unit (RSU) activity during the six months ended June 30, 2016:

	Shares			Weighted-Average Exercise Price		Weighted-AverageRemainingContractual Term (Years)		AggregateIntrinsicValue (in thousands)
Outstanding, December 31, 2015		4,146,064		$	16.90		7.4
Granted		206,174			17.60
Exercised		(350,147	)		1.12
Forfeited or expired		(94,538	)		28.70
Outstanding, June 30, 2016		3,907,553			18.06		7.4		$	41,198
As of June 30, 2016:
Exercisable		2,076,701			11.75		6.2			33,306
Vested and expected to vest		3,689,857			17.71		7.3			40,055

The weighted-average grant-date fair value of options granted for the six months ended June 30, 2016 was $12.78. The total intrinsic value of options exercised during the six months ended June 30, 2016 was approximately $7.2 million, and the total cash received for options exercised was approximately $0.4 million. The total fair value of shares vested in the six months ended June 30, 2016 was approximately $6.0 million. As of June 30, 2016, the total unrecognized compensation expense related to non-vested stock options, net of related forfeiture estimates, was approximately $25.8 million, which the Company expects to recognize over a weighted-average period of approximately three years.