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Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2021

Share-based Payment Arrangement [Abstract]

Schedule of Stock Option Activity

The following table summarizes activity under the 2008 Plan and the 2014 Plan, including grants issued to nonemployees, in the year ended December 31, 2021:

			Shares Available for Grant						Options Issued and Outstanding												Weighted Average Remaining Contractual Term						Aggregate Intrinsic Value
									Number of Shares						Weighted-Average Exercise Price  per Share
																					(in Years)						(in thousands)
Balance at December 31, 2020			1,757,058						9,790,049						$	6.76
Options authorized			4,201,766						—						$	—
Options granted			(3,409,719)						3,409,719						$	6.60
Options exercised			—						(331,310)						$	1.96
Options canceled			2,451,306						(2,451,306)						$	6.17
Issuance of common stock for services			(25,989)						—						—
Balance at December 31, 2021			4,974,422						10,417,152						$	7.00					6.57						$	—
Vested and expected to vest at December 31, 2021									10,417,152						$	7.00					6.57						$	—
Exercisable at December 31, 2021									6,772,289						$	7.39					5.58						$	—

Schedule of Valuation Assumptions for Stock Options

The estimated grant date fair value of employee stock options was calculated using the Black-Scholes option-pricing model, based on the following weighted-average assumptions:

			Year Ended December 31,
			2021						2020						2019
Expected term (years)			4.97						6.00						6.00
Expected volatility			77		%				83		%				81		%
Risk-free interest rate			4.69		%				1.07		%				2.42		%
Dividend yield			—		%				—		%				—		%

Schedule of Restricted Stock Units Activity

The following table summarizes restricted stock unit activity under the 2014 Plan in the year ended December 31, 2021, and includes restricted stock units with time or service-based vesting and those restricted stock units with performance-based vesting:

			Number of RSUs						Weighted-Average Grant Date Fair Value Per Share
Non-vested restricted stock units at December 31, 2020			158,626						$	5.64
Granted			4,144,051						$	2.71
Vested			(193,147)						$	6.39
Forfeited			(580,848)						$	6.38
Non-vested restricted stock units at December 31, 2021			3,528,682						$	2.04

Schedule of Employee Stock Purchase Plan Activity

The following table summarizes our ESPP activity during the year ended December 31, 2021:

			Shares Available for Grant						Number of Shares Purchased						Average Purchase Price per Share						Gross Proceeds
																					(in thousands)
Balance at December 31, 2020			349,647						661,611
Shares purchased			(386,664)						386,664						$	2.12					$	819
Balance at December 31, 2021			898,982						1,048,275

Schedule of Valuation Assumptions for Employee Stock Purchase Plan

The following table illustrates the weighted-average assumptions for the Black-Scholes option-pricing model used in determining the fair value of ESPP purchase rights granted to our employees:

			Year Ended December 31,
			2021						2020						2019
Expected term (years)			0.50						0.50						0.50
Expected volatility			123		%				79		%				69		%
Risk-free interest rate			0.65		%				0.48		%				2.00		%
Dividend yield			—		%				—		%				—		%

Schedule of Stock-Based Compensation Recognized

Total stock-based compensation recognized was as follows (in thousands):

			Year Ended December 31,
			2021						2020						2019
Research and development			$	4,116					$	4,061					$	4,104
General and administrative			7,923						6,522						5,832
Total			$	12,039					$	10,583					$	9,936

Schedule of Total Unrecognized Stock-Based Compensation Expense, Net of Estimated Forfeitures

At December 31, 2021, the Company had total unrecognized stock-based compensation expense, net of estimated forfeitures, of the following (dollars in thousands):

			December 31, 2021
			Unrecognized Compensation Expense						Average Remaining Vesting Period (Years)
Stock options grant			$	14,506					2.4
RSU grants			$	2,262					0.8
ESPP			$	44					0.1

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