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STOCKHOLDERS' EQUITY (Tables) | 12 Months Ended
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Dec. 31, 2022
Stockholders' Equity Note [Abstract] |
Schedule of aggregate stock option and award activity | Aggregate stock option and award activity for the 2011 Plan, 2018 Plan, 2006 Directors Plan, Inducement Plan and Directors Market Plan is as follows:
|  |  |  |  | Outstanding Stock Options |
|  |  |  |  |  |  |  |  |  |  | Weighted Average |  |  | Aggregate |
|  | Shares |  |  |  |  |  | Weighted Average |  |  | Remaining |  |  | Intrinsic |
|  | Available |  |  | Number of |  |  | Exercise Price |  |  | Contractual Life |  |  | Value |
|  | For Grant |  |  | Shares |  |  | Per Share |  |  | (In years) |  |  | (In thousands) |
Balance at December 31, 2021 |  |  | 18,763,393 |  |  |  | 49,952,409 |  |  | $ | 2.06 |  |  |  |  |  |  |
Additional shares authorized |  |  | 16,755,000 |  |  |  | — |  |  | $ | — |  |  |  |  |  |  |
Stock options granted |  |  | (19,747,760 | ) |  |  | 19,747,760 |  |  | $ | 1.37 |  |  |  |  |  |  |
Awards granted |  |  | (15,962 | ) |  |  | — |  |  | $ | — |  |  |  |  |  |  |
Stock options exercised |  |  | — |  |  |  | (1,181,711 | ) |  | $ | 1.52 |  |  |  |  |  |  |
Stock options cancelled/forfeited/expired |  |  | 2,616,058 |  |  |  | (2,616,058 | ) |  | $ | 1.70 |  |  |  |  |  |  |
Balance at December 31, 2022 |  |  | 18,370,729 |  |  |  | 65,902,400 |  | (1) | $ | 1.87 |  |  |  | 6.56 |  |  | $ | 50,205 |
Stock options exercisable at December 31, 2022 |  |  |  |  |  | 36,085,389 |  |  | $ | 2.17 |  |  |  | 5.09 |  |  | $ | 22,878 |
Stock options fully vested and expected to vest at December 31, 2022 |  |  |  |  |  | 64,539,649 |  |  | $ | 1.88 |  |  |  | 6.52 |  |  | $ | 48,951 |
(1) Includes 8,791,750 performance-based stock options granted that have not achieved certain strategic milestones.
Summary of allocation of stock-based compensation expense related to share-based payment awards | We recognize stock‑based compensation expense for service-based stock options on a straight‑line basis over the requisite service period, which is generally the vesting period. We have not recognized any stock-based compensation expense for performance-based stock options on our consolidated statements of operations for the years ended December 31, 2022, 2021 and 2020, as the achievement of the specified strategic milestones was not considered probable during that time. The following table summarizes the stock‑based compensation expense related to service-based stock options and employee stock purchases for the years ended December 31, 2022, 2021 and 2020 which was allocated as follows:
|  | Year Ended December 31, |
(In thousands) |  | 2022 |  |  | 2021 |  |  | 2020 |
Research and development |  | $ | 3,720 |  |  | $ | 3,597 |  |  | $ | 2,337 |
General and administrative |  |  | 4,281 |  |  |  | 4,483 |  |  |  | 4,558 |
Stock-based compensation expense included in operating expenses |  | $ | 8,001 |  |  | $ | 8,080 |  |  | $ | 6,895 |
Schedule of assumptions used to estimate the fair value of stock options granted | The fair value of stock options granted in 2022, 2021, and 2020 has been estimated at the date of grant using the Black Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2022 |  | 2021 |  | 2020
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.772 to 0.817 |  | 0.775 to 0.783 |  | 0.781 to 0.793
Risk-free interest rate range |  | 1.69% to 4.57% |  | 0.51% to 1.30% |  | 0.31% to 1.62%
Expected term range |  | 5.5 yrs |  | 5.5 yrs |  | 5.25 yrs
Schedule of assumptions used to estimate the fair value of employee stock purchases under the purchase plan | The fair value of employee stock purchases in 2022, 2021, and 2020 has been estimated using the Black Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2022 |  | 2021 |  | 2020
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.614 to 0.865 |  | 0.507 to 0.707 |  | 0.478 to 0.818
Risk-free interest rate range |  | 0.40% to 2.79% |  | 0.09% to 0.16% |  | 0.16% to 1.57%
Expected term range |  | 6 - 12 mos |  | 6 - 12 mos |  | 6 - 12 mos
Schedule of common stock reserved for future issuance | Common stock reserved for future issuance as of December 31, 2022 is as follows:
Outstanding stock options |  |  | 65,902,400 |
Stock options and awards available for grant |  |  | 18,370,729 |
Employee stock purchase plan |  |  | 1,131,764 |
Warrants outstanding |  |  | 106,254,842 |
Total |  |  | 191,659,735 |

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