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Stock-based Compensation

3 Months Ended

Mar. 31, 2021

13. Stock-based Compensation

Performance Share Units, Restricted Stock Units and Stock Options

Performance Share Units

The Company periodically grants PSUs to certain members of the Company's senior management team. PSUs vest subject to the satisfaction of annual and cumulative performance and/or market conditions established by the Compensation Committee.

A summary of the Company’s PSU activity for the three months ended March 31, 2021 and related information is as follows:


		Weighted-Average
	Shares	Grant Date Fair Value
Outstanding at December 31, 2020	283,223	$	24.26
Granted	213,180		35.12
Vested	(60,990)		22.45
Forfeited	—		—
Performance adjustment	(12,609)		21.80
Outstanding at March 31, 2021	422,804	$	30.07

The number of PSUs granted represents the target number of shares of common stock that may be earned, however, the actual number of shares earned may vary based on the satisfaction of performance criteria. The weighted-average grant date fair value of PSUs granted for the three months ended March 31, 2021 and 2020 was $35.12 and $28.49, respectively.

Restricted Stock Units

The Company granted RSUs to employees for the three months ended March 31, 2021. The Company’s RSUs generally vest ratably over a four-year period of service. A summary of the Company’s RSU activity for the three months ended March 31, 2021 and related information is as follows:


		Weighted-Average
	Shares	Grant Date Fair Value
Outstanding at December 31, 2020	1,242,387	$	19.42
Granted	1,163,664		24.43
Vested	(352,548)		19.42
Forfeited	(14,968)		21.35
Outstanding at March 31, 2021	2,038,535	$	22.27

The weighted-average grant date fair value per share of RSUs granted for the three months ended March 31, 2021 and 2020 was $24.43 and $21.34, respectively. The total fair value of RSUs vested (measured on the date of vesting) for the three months ended March 31, 2021 and 2020 was $9,116 and $4,204, respectively.

Stock Options

A summary of the Company’s stock option activity for the three months ended March 31, 2021 and related information is as follows:


				Weighted-
			Weighted-	Average
			Average	Remaining		Aggregate
			Exercise Price	Contractual		Intrinsic
	Shares		per Share	Term (in years)		Value
Outstanding at December 31, 2020	3,860,481	$	17.78	7.2	$	13,011
Granted	63,000		19.97
Exercised	(289,164)		14.18
Cancelled	(25,491)		19.30
Outstanding at March 31, 2021	3,608,826	$	18.09	7.0	$	20,895
Exercisable at March 31, 2021	2,353,069	$	17.57	6.3	$	14,949

The weighted average assumptions used in the Black Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:


	Three months ended March 31,
	2021		2020
Risk-free interest rate	0.5	%	1.5	%
Volatility	68.3	%	65.5	%
Expected term (years)	6.0		6.1
Expected dividend yield	—	%	—	%

The weighted-average grant date fair value per share of stock options granted for the three months ended March 31, 2021 and 2020 was $12.08 and $12.86, respectively.

Employee Stock Purchase Plan

The Company’s 2015 Employee Stock Purchase Plan allows employees to purchase shares of the Company’s common stock. The purchase price is equal to 85% of the lower of the closing price of the Company’s common stock on (1) the first day of the purchase period or (2) the last day of the purchase period. During the three months ended March 31, 2021, 24,630 shares of common stock were purchased for total proceeds of $358. The expense for the three months ended March 31, 2021 and 2020 was $72 and $79, respectively.

Stock-based Compensation Expense

A summary of the Company’s stock-based compensation expense was allocated as follows:


	Three months ended March 31,
	2021		2020
Research and development	$	1,217	$	770
Selling, general and administrative		5,662		4,181
Total stock-based compensation expense	$	6,879	$	4,951

At March 31, 2021, there was approximately $63,922 of unrecognized compensation expense related to unvested options, restricted stock units and performance stock units, which is expected to be recognized as expense over a weighted average period of approximately 2.9 years.