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Share-Based Compensation - (Tables)
 6 Months Ended
Jun. 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 June 30,Six Months Ended June 30,
 2021202020212020
Stock options$1,042 $1,093 $2,511 $2,467 
Restricted stock units1,003 752 1,795 1,688 
Employee stock purchase plan34 28 65 42 
Total$2,079 $1,873 $4,371 $4,197 
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 June 30,Six Months Ended June 30,
 2021202020212020
Research and development$1,319 $951 $2,662 $2,104 
General and administrative760 922 1,709 2,093 
Total$2,079 $1,873 $4,371 $4,197 
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 June 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 
Granted549 $6.66 
Exercised(144)$3.18 
Forfeited(100)$4.91 
Outstanding, June 30, 20219,233 $9.47 
Vested and exercisable, June 30, 20215,148 $12.67 
Vested and expected to vest, June 30, 20218,903 $9.62 
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 June 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 
Awarded201 $6.57 
Vested(98)$3.31 
Forfeited(108)$36.14 
Outstanding, June 30, 20211,953 $5.48 
Outstanding, Expected to vest1,795 $5.48