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Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

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,
​	​	​	2020		​	2019
Research and development	​	​	$	11,580	​	$	13,538
General and administrative	​	​		11,576	​		13,705
Total stock-based compensation expense	​	​	$	23,156	​	$	27,243

​

Schedule of changes in unvested restricted stock

​

​	​	​	​	​	​
​		​		Weighted
​	​	​	​	Average
​	​	​	​	Grant Date
​	​	​	​	Fair Value
​	​	Shares	​	Per Share
Unvested restricted stock and restricted stock unit awards as of December 31, 2019		581,408	​	$	24.03
Issued		364,549	​	$	29.57
Vested		(304,638)	​	$	23.37
Forfeited	​	(133,869)	​	$	24.72
Unvested restricted stock and restricted stock unit awards as of December 31, 2020		507,450	​	$	27.35

Schedule of stock option activity

​

​	​	​	​	​	​	​	​	​	​	​
​		​		Weighted Average			Remaining		Aggregate Intrinsic
​	​	Shares	​	Exercise Price		​	Contractual Life (years)	​	Value (in thousands)
Outstanding at December 31, 2019		4,358,291	​	$	25.40	​	7.4	​	$	26,060
Granted	​	1,721,748	​	$	30.44	​	​	​	​	​
Exercised	​	(964,412)	​	$	20.22	​	​	​	​	​
Cancelled	​	(1,203,370)	​	$	30.72	​	​	​	​	​
Outstanding at December 31, 2020		3,912,257	​	$	27.26	​	7.9	​	$	167,640
Exercisable at December 31, 2020		1,463,668	​	$	24.05	​	6.8	​	$	67,413

Schedule of assumptions used to value stock options

​

​	​	​	​	​	​	​	​	​	​
​		Year Ended							​
​	​	December 31,							​
​	​	2020		​	2019			2018
Expected volatility		60.0	%	​	73.8	%	​	77.5	%
Expected option term (in years)		6.25	​	​	6.25	​	​	6.25	​
Risk free interest rate		1.5	%	​	2.0	%	​	2.9	%
Expected dividend yield		—	​	​	—	​	​	—	​

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Tabular disclosure of the changes in outstanding nonvested restricted stock shares.

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