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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2021
Share-based Payment Arrangement [Abstract]  
Stock Option Valuation The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees, directors and non-employees.
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
Risk-free interest rate1.1 %0.4 %1.0 %0.6 %
Expected term (in years)6.16.16.06.0
Expected volatility95.7 %95.1 %97.6 %94.9 %
Expected dividend yield0 %0 %0 %0 %
Summary of Stock Option Activity The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2021:
Number of
Shares		Weighted
Average
Exercise
Price		Weighted
Average
Contractual
Term (Years)		Aggregate
Intrinsic
Value (in thousands)
Outstanding as of December 31, 20201,874,514 $12.94 8.3$7 
Granted336,750 6.98 
Exercised(5,860)7.08 
Forfeited and Expired(252,513)15.21 
Outstanding as of September 30, 20211,952,891 $11.64 7.9$9 
Exercisable as of September 30, 2021919,423 $15.45 6.7$— 
Vested and expected to vest as of September 30, 20211,702,001 $11.92 7.8$5 
Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity
Restricted Stock Units— The following table summarizes the Company's restricted stock unit activity for the nine months ended September 30, 2021:
Number of
Shares
Unvested as of December 31, 2020572,460 
Granted649,478 
Vested(205,185)
Forfeited(109,880)
Unvested as of September 30, 2021906,873 
Summary of Stock-Based Compensation Expense Classification Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows:
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
Research and development expense$633 $1,006 $2,008 $1,711 
General and administrative expense883 1,238 2,563 2,319 
Total stock-based compensation$1,516 $2,244 4,571 4,030