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STOCKHOLDERS' EQUITY (Tables)

12 Months Ended

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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Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement.

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