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STOCK-BASED COMPENSATION (Tables)
12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Stock-Based Compensation Expense The following table shows total stock-based compensation expense recognized in the accompanying Consolidated Statements of Operations (in thousands):
Year Ended December 31,
202120202019
Research and development$19,534 $13,523 $10,135 
General and administrative13,422 12,185 9,195 
Total stock-based compensation expense$32,956 $25,708 $19,330 
Assumptions Used for Estimating Fair Value of Employee Stock Options The assumptions used for estimating the fair value of the employee stock options were as follows:
Year Ended December 31,
202120202019
Risk-free interest rate
0.95-1.22%
0.34-0.61%
1.68-2.25%
Expected term (in years)
5.46-5.52
5.51-5.57
5.50-5.62
Expected dividend yield of stock— — — 
Expected volatility
77.30-79.77%
77.61-80.32%
76.46-78.39%
Weighted-Average Assumptions Used for Estimating Fair Value of ESPP Purchase Rights The assumptions used for estimating the fair value of the ESPP purchase rights are as follows:
Year Ended December 31,
202120202019
Risk-free interest rate
0.01-2.80%
1.53-2.80%
1.53-2.42%
Expected term (in years)
0.01-2.0
0.5-2.0
0.5-2.0
Expected dividend yield of stock— — — 
Expected volatility
32.54-97.88%
51.02-91.96%
51.02-91.96%