STOCKHOLDERS' EQUITY (Tables)

12 Months Ended

Dec. 31, 2023

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
	Shares							Weighted Average		Remaining		Aggregate
	Available			Number of				Exercise Price		Contractual Life		Intrinsic
	For Grant			Shares				Per Share		(In years)		Value
Balance at December 31, 2022		18,370,729			65,902,400			$	1.87
Additional shares authorized		56,368,058			—			$	—
Stock options granted		(20,855,230	)		20,855,230			$	2.72
Awards granted		(36,864	)		—			$	—
Stock options exercised		—			(8,869,302	)		$	1.39
Stock options cancelled/forfeited/expired		4,903,977			(4,903,977	)		$	2.15
Balance at December 31, 2023		58,750,670			72,984,351		(1)	$	2.16		6.70	$	25,391,643
Stock options exercisable at   December 31, 2023					39,995,642			$	2.16		5.28	$	15,557,976
Stock options fully vested and expected     to vest at December 31, 2023					71,983,176			$	2.15		6.67	$	25,169,074

(1)Includes 7,936,030 performance-based stock options granted that have not achieved the specified performance milestones.

Summary of allocation of stock-based compensation expense related to share-based payment awards

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, 2023, 2022 and 2021, which was allocated as follows:

	Year Ended December 31,
(In thousands)	2023		2022		2021
Research and development	$	7,426	$	3,720	$	3,597
General and administrative		11,099		4,281		4,483
Stock-based compensation expense    included in operating expenses	$	18,525	$	8,001	$	8,080

Schedule of assumptions used to estimate the fair value of stock options granted

The fair value of stock options granted in 2023, 2022, and 2021 has been estimated at the date of grant using the Black-Scholes option‑pricing model with the following assumptions:

	Year Ended December 31,
	2023	2022	2021
Dividend yield	0%	0%	0%
Expected volatility range	0.815 to 0.827	0.772 to 0.817	0.775 to 0.783
Risk-free interest rate range	3.42% to 4.94%	1.69% to 4.57%	0.51% to 1.30%
Expected term range	6.0 yrs	5.5 yrs	5.5 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 2023, 2022, and 2021 has been estimated using the Black-Scholes option‑pricing model with the following assumptions:

	Year Ended December 31,
	2023	2022	2021
Dividend yield	0%	0%	0%
Expected volatility range	0.791 to 0.832	0.614 to 0.865	0.507 to 0.707
Risk-free interest rate range	4.73% to 5.4%	0.40% to 2.79%	0.09% to 0.16%
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, 2023 is as follows:

Outstanding stock options	72,984,351
Stock options and awards available for grant	58,750,670
Employee stock purchase plan	745,838
Warrants outstanding	53,904,980
Total	186,385,839