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Stock-Based Compensation
 3 Months Ended
Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Stock-Based Compensation STOCK-BASED COMPENSATION
As of March 31, 2024, there are an aggregate of approximately 4.0 million shares of common stock available for issuance under the Company’s equity incentive plans. Approximately 4.9 million shares of common stock remain available for issuance under the 2017 ESPP.

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 March 31,
20242023
Risk-free interest rate4.1 %3.6 %
Expected term (in years)6.16.0
Expected volatility95.7 %90.8 %
Expected dividend yield0 %0 %
Stock Options
The following table summarizes the Company’s stock option activity for the three months ended March 31, 2024:
Number of
Shares		Weighted
Average
Exercise
Price		Weighted
Average
Contractual
Term (Years)		Aggregate
Intrinsic
Value (in thousands)
Outstanding as of December 31, 20236,008,541 $2.97 8.6$24 
Granted2,387,467 0.93 
Forfeited and Expired(313,170)2.11 
Outstanding as of March 31, 20248,082,838 $2.40 8.8$2,123 
Exercisable as of March 31, 20241,535,408 $7.44 6.0$113 
Vested and expected to vest as of March 31, 20246,136,867 $2.79 8.6$1,501 
 
The weighted average grant-date fair value per share of stock options granted during the three months ended March 31, 2024 and 2023 was $0.73 and $0.69, respectively.

Restricted Stock Units— The following table summarizes the Company's restricted stock unit activity for the three months ended March 31, 2024:
Number of
Shares
Unvested as of December 31, 20233,118,824 
Granted5,882,459 
Vested(503,186)
Forfeited(126,511)
Unvested as of March 31, 20248,371,586 
During the three months ended March 31, 2024, the Company granted performance-based restricted stock units (“PRSUs”) to its employees. The PRSUs vest 50% based on the Company’s achievement of each of two operational milestones conditioned on the grantee’s continued employment with the Company. As of March 31, 2024, neither of the two performance criteria had been met. Stock-based compensation expense has been recognized for awards for which vesting is considered probable using the accelerated attribution model based on the fair value of the awards as of the date of grant and management’s best estimate of
the date the probable operational milestone will be achieved. The Company updates its estimates related to the probability and timing of achievement of the operational milestones each period until the award either vests or is forfeited.
Stock-Based Compensation— As of March 31, 2024, total unrecognized compensation expense related to unvested stock options and restricted stock units was $6.5 million, which is expected to be recognized over a weighted average period of 2.4 years.
Stock-based compensation expense was classified in the condensed consolidated statements of operations and comprehensive loss as follows:
Three Months Ended March 31,
(in thousands)20242023
Research and development expense$783 $831 
Selling, general and administrative expense956 814 
Total stock-based compensation$1,739 $1,645