Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2015

Schedule of Stock-Based Compensation Expense

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

	Three Months Ended September 30,						Nine Months Ended September 30,
	2015			2014			2015			2014
Research and development		$	916		$	380		$	2,607		$	1,062
General and administrative			1,182			476			3,024			1,187
Total stock-based compensation expense		$	2,098		$	856		$	5,631		$	2,249

Schedule of Fair Value Option Award Estimated on the Date of Grant Using the Black-Scholes Option-Pricing Model

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:

	Nine Months Ended September 30,
	2015	2014
Expected dividend yield	0%	0%
Expected volatility	74 % - 75 %	67 %
Risk-free interest rate	1.6% - 2.1%	2.0% - 2.3%
Expected term	6.25 years	6.25 years

Schedule of Stock Option Activity

The following table summarizes stock option activity under the Plan during the nine months ended September 30, 2015:

	Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2014		3,572,116		$	11.40		7.3
Granted		199,438			32.98
Exercised		(276,700	)		2.40
Forfeited or expired		(31,874	)		22.25
Outstanding, September 30, 2015		3,462,980			13.26		6.9		$	37,397
As of September 30, 2015:
Exercisable		1,898,382			6.58		5.6			29,810
Vested and expected to vest		3,261,836			12.89		6.9			36,179