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Equity Incentive Plans (Tables)
 9 Months Ended
Sep. 30, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Stock-Based Compensation Expense Recognized
Stock-based compensation expense recognized for stock options, restricted stock units ("RSUs"), performance-based restricted stock units ("PRSUs") and our employee stock purchase program (the "ESPP") are recorded as operating expenses in our condensed statements of operations and comprehensive loss, as follows (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
Research and development$730 $963 $2,909 $3,165 
General and administrative1,532 1,564 5,659 4,984 
Total$2,262 $2,527 $8,568 $8,149 
Summary of Total Unrecognized Stock-Based Compensation Expense, Net of Estimated Forfeitures
As of September 30, 2021, our total unrecognized stock-based compensation expense, net of estimated forfeitures, and average remaining vesting period, included the following (dollars in thousands):
Unrecognized Compensation
 Expense		Average Remaining
 Vesting Period (Years)
Stock options$19,546 2.6
RSUs$4,839 3.3
ESPP$201 0.4
Schedule of Stock Option Activity
A summary of our stock option activity and related information for the nine months ended September 30, 2021 is as follows (in thousands, except dollar amounts):
Number of SharesWeighted Average Exercise Price
Outstanding balance at December 31, 20209,790 $6.76 
Granted3,410 $6.60 
Exercised(331)$1.96 
Forfeited(1,481)$6.01 
Outstanding balance at September 30, 202111,387 $6.95 
Exercisable at September 30, 20216,434 $7.51 
Schedule of Restricted Stock Units Activity
A summary of our RSUs activity and related information for the nine months ended September 30, 2021 is as follows (in thousands, except dollar amounts):
Number of RSUsWeighted Average Value at Grant
Outstanding balance at December 31, 2020159 $5.64 
Granted1,124 $6.62 
Vested(140)$6.57 
Forfeited(322)$6.68 
Unvested at September 30, 2021820 $6.42