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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Stock Option Activity
The following table summarizes stock option activity for the 2002 Plan and 2018 Plan for the year ended December 31, 2021:
Total
Options
Weighted-
Average
Exercise
Price Per
Share
Weighted-
Average
Remaining
Contractual
Term (in years)
Aggregate
Intrinsic
Value
Outstanding at December 31, 20192,577,718 $3.29 7.9$7,914,459 
Granted
1,227,159 $5.09 
Exercised
(51,572)$1.85 
Cancelled or expired
(155,145)$4.89 
Outstanding at December 31, 20203,598,160 $3.85 7.5$1,737,432 
Granted
1,271,836 $3.91 
Exercised
(209,751)$1.88 
Cancelled or expired
(193,444)$4.53 
Outstanding at December 31, 20214,466,801 $3.93 7.2$700,875 
Vested and expected to vest at December 31, 20214,466,801 $3.93 7.2$700,875 
Vested and exercisable at December 31, 20212,559,327 $3.75 6.2$676,046 
Estimated Fair Value of Option on Grant Date using Weighted Average Assumptions
The fair value of each option was estimated on the date of grant using the weighted-average assumptions in the table below:
Year Ended December 31,
20212020
Risk-free interest rate
0.88 %1.37 %
Expected term (in years)
6.06.0
Expected volatility
72 %71 %
Expected dividend yield
— — 
Fair value of common stock
$3.91 $5.09 
Stock-based Compensation Expense
Stock-based compensation expense has been reported in the Company’s statements of operations for the years ended December 31, 2021 and 2020 as follows (in thousands):
Year Ended December 31,
20212020
General and administrative$1,509 $1,594 
Research and development759 643 
Total stock-based compensation$2,268 $2,237