Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

Schedule of Stock-Based Compensation Expense

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

	Year Ended December 31,
	2015			2014			2013

Research and development		$	3,623		$	1,562		$	507
General and administrative			4,224			1,682			355
Total stock-based compensation expense		$	7,847		$	3,244		$	862

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

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:

	Year Ended December 31,
	2015		2014		2013

Expected dividend yield		0%		0%		0%
Expected volatility		73% - 75%		67%		53% - 67%
Risk-free interest rate		1.6% - 2.1%		1.8% - 2.3%		1.2% - 2.2%
Expected term	6.25 years		6.25 years		7 years

Schedule of Stock Option and Restricted Stock Unit Activity

The following table summarizes stock option and restricted stock unit (RSU) activity for 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		987,680			31.59
Exercised		(374,214	)		2.45
Forfeited or expired		(39,518	)		23.44
Outstanding, December 31, 2015		4,146,064			16.90		7.4		$	59,344

December 31, 2015:
Exercisable		1,992,951			7.94		5.7			46,012
Vested and expected to vest		3,894,148			16.50		7.3			57,267