Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Stock-based Compensation

Schedule of stock-based compensation expense

Total compensation cost recognized for all stock‑based compensation awards in the consolidated statements of operations was as follows (in thousands):

	Year Ended
	December 31,
	2017		2016		2015
Research and development	$	15,131	$	4,234	$	3,015
General and administrative		8,233		12,647		498
Total stock-compensation expense	$	23,364	$	16,881	$	3,513

Schedule of changes in unvested restricted stock

		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested Restricted Common Stock as of December 31, 2016	822,638	$	0.02
Issued	480,000	$	28.05
Vested	(784,119)	$	4.95
Forfeited	(5,294)	$	0.03
Unvested Restricted Common Stock as of December 31, 2017	513,225	$	18.70

Schedule of stock option activity

		Weighted Average		Remaining	Aggregate Intrinsic
	Shares	Exercise Price		Contractual Life	Value (in thousands)
Outstanding at December 31, 2016	3,411,783	$	13.71	8.8	$	16,190
Granted	1,392,689		—	—		—
Exercised	(289,583)		—	—		—
Cancelled	(142,753)		—	—		—
Outstanding at December 31, 2017	4,372,136	$	17.28	8.5	$	60,591
Vested and expected to vest at December 31, 2017	4,372,126	$	17.28	8.5	$	60,591
Exercisable at December 31, 2017	1,540,023	$	14.95	8.3	$	25,099

Schedule of assumptions used to value stock options

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions:

	Year Ended
	December 31,
	2017		2016		2015
Expected volatility	77.8	%	78.4	%	78.8	%
Expected term (in years)	6.25		6.25		6.25
Risk free interest rate	2.1	%	1.5	%	1.7	%
Expected dividend yield	—		—		—

There were no options granted to persons other than employees and directors during the year ended December 31, 2017. For the year ended December 31, 2017, 2016 and 2015, the fair value of each option issued to persons other than employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the weighted‑average assumptions set forth in the table below:

	Year Ended
	December 31,
	2017	2016		2015
Expected volatility	—	76.5	%	80.0	%
Expected term (in years)	—	10.0		10.0
Risk free interest rate	—	1.6	%	2.2	%
Expected dividend yield	—	—		—