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Share-Based Compensation - (Tables)
 9 Months Ended
Sep. 30, 2021
Share-based Payment Arrangement [Abstract]  
Schedule of Allocation of Share-based Compensation Costs by Award Type Stock-based compensation by award type was as follows:
 Three Months Ended September 30,Nine Months Ended September 30,
 2021202020212020
Stock options$1,485 $1,089 $3,997 $3,556 
Restricted stock units1,101 723 2,896 2,411 
Employee stock purchase plan40 35 104 77 
Total$2,626 $1,847 $6,997 $6,044 
Schedule of Allocation of Share-based Compensation Costs by Financial Statement Line
The Company allocated stock-based compensation expense in the Company’s Consolidated Statements of Operations and Comprehensive Loss as follows:
 Three Months Ended September 30,Nine Months Ended September 30,
 2021202020212020
Research and development$1,476 $882 $4,138 $2,986 
General and administrative1,150 965 2,859 3,058 
Total$2,626 $1,847 $6,997 $6,044 
Summary of the stock option activity including weighted average exercise prices, options outstanding and exercisable
A summary of the stock option activity, weighted average exercise prices, options outstanding, exercisable and expected to vest as of September 30, 2021 is as follows (in thousands except weighted average exercise price):
 Number of Shares Underlying OptionsWeighted
Average
Exercise
Price
Outstanding, December 31, 20208,928 $9.22 
Granted969 $7.84 
Exercised(487)$2.89 
Forfeited(321)$4.49 
Outstanding, September 30, 20219,089 $9.85 
Vested and exercisable, September 30, 20215,154 $12.71 
Vested and expected to vest, September 30, 20218,768 $9.99 
Summary of the Company's outstanding shares of RSU awards
The following table presents a summary of the Company's outstanding RSU awards granted as of September 30, 2021 (in thousands except weighted average grant-date fair value):
Restricted
Stock
Units		Weighted Average
Grant-Date
Fair Value
Outstanding, December 31, 20201,958 $6.95 
Awarded329 $7.44 
Vested(181)$4.32 
Forfeited(203)$21.60 
Outstanding, September 30, 20211,903 $5.76 
Outstanding, Expected to vest1,750 $5.76