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Stock Plans and Stock-Based Compensation (Tables)
12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Stock Option Grants, Including Performance Options
The following table summarizes activity in the Company’s stock option grants:
 
    
Shares
Subject to
Outstanding
Options
    
Weighted-
Average
Exercise
Price of
Options
    
Weighted-
Average
Remaining
Contractual
Term
(Years)
    
Aggregate

Intrinsic
Value (in
thousands)
 
Outstanding as of December 31, 2020
     8,811,630      $ 6.35                    
Options granted
     2,660,965        4.84                    
Options exercised
     (106,847      2.05                    
Options forfeited
     (307,018      5.89                    
Options expired
     (267,299      7.60                    
    
 
 
                            
Outstanding as of December 31, 2021
     10,791,431      $ 6.01        7.28      $ 1,514  
    
 
 
                            
Vested and expected to vest as of December 31, 2021
     10,791,431      $ 6.01        7.28      $ 1,514  
    
 
 
                            
Exercisable as of December 31, 2021
     6,327,680      $ 6.62        6.22      $ 1,510  
    
 
 
                            
Summary of Stock-Based Compensation Expense
Stock-based compensation expense is included in the consolidated statements of operations and comprehensive loss and is as follows (in thousands):
 
    
Year Ended
December 31,
 
    
2021
    
2020
 
Research and development
   $ 4,470      $ 2,739  
General and administrative
     5,526        4,585  
    
 
 
    
 
 
 
Total stock-based compensation expense
   $ 9,996      $ 7,324  
    
 
 
    
 
 
 
Estimated Weighted-Average Grant Date Fair Value
The following table presents the weighted-average assumptions the Company used in the Black-Scholes option-pricing model to derive the grant date fair values of stock options granted in each of the years presented along with the resulting estimated weighted-average grant date fair values per share:
 
    
Year Ended
December 31,
 
    
2021
   
2020
 
Expected term (years)
     6.1       6.1  
Expected volatility
     104     105
Risk-free interest rate
     0.9     0.4
Expected dividend yield
     —         —    
Weighted-average grant date fair value per share
   $ 3.91     $ 3.91