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STOCKHOLDERS' EQUITY (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of aggregate option and award activity

Aggregate option and award activity for the 2002 Plan, 2011 Plan, 2018 Plan, 2006 Directors Plan, Inducement Plan and Directors Market Plan is as follows:

					Outstanding 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, 2019		6,407,355				37,614,013			$	2.26
Additional shares authorized		7,000,000				—			$	—
Options granted		(6,531,931	)			6,531,931			$	1.37
Awards granted		(17,986	)			—			$	—
Options exercised		—				(21,944	)		$	1.16
Options cancelled/forfeited/expired		400,309				(460,309	)		$	3.48
Balance at December 31, 2020		7,257,747		⁽¹⁾		43,663,691		⁽²⁾	$	2.12		6.32	$	5,624
Options exercisable at December 31, 2020						25,721,508			$	2.54		4.90	$	2,247
Options fully vested and expected to vest at December 31, 2020						42,621,956			$	2.13		6.27	$	5,437

(1)	In February 2021, our Compensation Committee approved an amendment to increase the reserve for the Inducement Plan from 9,300,000 to 10,100,000 shares of common stock.

(2)	Includes 5,500,000 performance-based stock options granted previously that have not achieved certain strategic 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, restricted stock awards and employee stock purchases for the years ended December 31, 2020, 2019 and 2018 which was allocated as follows:

	Year Ended December 31,
(In thousands)	2020		2019		2018
Research and development	$	2,337	$	1,640	$	949
General and administrative		4,558		4,439		5,419
Stock-based compensation expense included in operating expenses	$	6,895	$	6,079	$	6,368

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

The fair value of stock options granted in 2020, 2019 and 2018 has been estimated at the date of grant using the Black Scholes option‑pricing model with the following assumptions:

	Year Ended December 31,
	2020	2019	2018
Dividend yield	0%	0%	0%
Expected volatility range	0.781 to 0.793	0.792 to 0.980	0.821 to 0.990
Risk-free interest rate range	0.31% to 1.62%	1.50% to 2.56%	2.55% to 3.11%
Expected term range	5.25 yrs	5.25 - 6.44 yrs	5.25 - 6.62 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 2020, 2019 and 2018 has been estimated using the Black Scholes option‑pricing model with the following assumptions:

	Year Ended December 31,
	2020	2019	2018
Dividend yield	0%	0%	0%
Expected volatility range	0.478 to 0.818	0.646 to 1.653	0.437 to 0.475
Risk-free interest rate range	0.16% to 1.57%	1.94% to 2.63%	1.53% to 1.76%
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, 2020 is as follows:

Outstanding stock options			43,663,691
Options and awards available for grant			7,257,747
Employee stock purchase plan			682,508
Warrants outstanding			66,552,939
Total			118,156,885