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EQUITY INCENTIVE PLANS (Tables)
12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Schedule of Restricted Stock Units and Performance-based Restricted Stock Units Activity
The following table summarizes RSU and PRSU activity during the year ended December 31, 2023:
Number of
RSUs and PRSUs
Weighted Average Grant Date Fair Value
Outstanding at December 31, 2022
1,223,871 $1.47 
RSUs and PRSUs granted1,515,817 1.00 
RSUs and PRSUs vested(490,520)1.48 
RSUs and PRSUs canceled(151,115)1.29 
Outstanding at December 31, 2023
2,098,053 $1.14 
Schedule of Stock Option Activity
The following table summarizes stock option activity during the year ended December 31, 2023:
Number of
Shares
Weighted
Average
Exercise Price
Weighted
Average
Remaining
Contractual Life
in Years
Total Aggregate
Intrinsic Value (in thousands)
Outstanding at December 31, 2022
8,099,624 $2.78 6.65$48 
Options granted3,233,558 1.02  
Options exercised(25,750)0.83  
Options canceled(641,417)3.12  
Outstanding at December 31, 2023
10,666,015 $2.23 6.72$57 
Vested and expected to vest at December 31, 2023
10,666,015 $2.23 6.72$57 
Exercisable at December 31, 2023
7,236,987 $2.75 5.71$54 
Schedule of Black-Scholes Option Pricing Model Assumptions
The following table summarizes the Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted to employees under the 2015 EIP and 2020 IIP and the shares purchasable under the 2015 ESPP during the periods presented:
 For the years ended December 31,
 20232022
2015 EIP and 2020 IIP  
Risk-free interest rate
3.58% - 4.61%
1.39% - 4.02%
Expected dividend yield
0%
0%
Expected volatility
82% - 83%
70% - 75%
Expected term (years)
5.50 - 6.08
5.50 - 6.08
2015 ESPP
Risk-free interest rate
4.29% - 5.43%
1.57% - 2.65%
Expected dividend yield
0%
0%
Expected volatility
64% - 147%
75% - 98%
Expected term (years)
0.50 - 2.00
0.50 - 2.00
Schedule of Stock-Based Compensation Expense
Stock-based compensation expense recognized for RSUs, PRSUs, stock options, and the ESPP has been reported in the consolidated statements of operations and comprehensive loss as follows (in thousands):
Years ended December 31,
 20232022
Research and development$1,592 $1,760 
Selling, general and administrative1,469 1,945 
Total$3,061 $3,705