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Equity Plans (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of activity under equity incentive plans



				Weighted-			Weighted-
				Average			Average
				Exercise			Remaining		Aggregate
		Options		Price Per			Contractual		Intrinsic
		Outstanding		Share			Life (years)		Value (1)
									(in millions)
Balances at December 31, 2019		3,681,521		$	11.64		7.78		$	2.4
Options granted		1,536,400			11.53
Options exercised		(294,238)			7.49
Options forfeited		(275,563)			11.54
Balances at December 31, 2020		4,648,120		$	11.87		7.61		$	40.0
Options exercisable – December 31, 2020		2,619,943		$	12.10		6.65		$	22.1
Options vested and expected to vest – December 31, 2020		4,648,120		$	11.87		7.61		$	40.0

____________________

⁽¹⁾The aggregate intrinsic values were calculated as the difference between the exercise price of the options and the closing price of the Company’s common stock on December 31, 2020. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on December 31, 2020.

Schedule of stock-based compensation expense

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


		Year Ended December 31,
		2020			2019			2018
Research and development		$	4,121		$	4,350		$	3,424
General and administrative			3,778			4,003			3,495
Total stock-based compensation expense		$	7,899		$	8,353		$	6,919

Options to purchase common stock

Black-Scholes option-pricing model assumptions


		Year Ended December 31,
		2020		2019		2018
Expected term (in years)		5.27 - 6.08		5.00 - 6.08		5.49 - 6.08
Expected volatility		72.1% - 87.5%		61.0% - 64.8%		62.0% - 66.5%
Risk-free interest rate		0.23% - 1.44%		1.42% - 2.58%		2.42% - 3.03%
Dividend yield		—		—		—

Restricted stock units

Schedule of activity under equity incentive plans


				Weighted
				Average
		Number of		Grant Date
		Shares		Fair Value

Unvested at December 31, 2019		278,482		$	10.08
Granted		142,000			7.80
Vested		(131,147)			9.52
Forfeited		(44,790)			8.67
Unvested at December 31, 2020		244,545		$	9.31

Black-Scholes option-pricing model assumptions


		Year Ended December 31,
		2020		2019		2018
Expected term (in years)		0.50		0.50		0.50
Expected volatility		89.1% - 120.4%		58.9% - 65.3%		49.0% - 63.4%
Risk-free interest rate		0.12% - 0.43%		1.89% - 2.32%		1.89% - 2.32%
Dividend yield		—		—		—