Share-based compensation is accounted for in accordance with the provisions of ASC 718, Compensation-Stock Compensation.

The Company has granted stock option awards to employees, directors and consultants from its 2011 Stock Incentive Plan (the “2011 Plan”) and its 2016 Equity Incentive Plan (the “2016 Plan”). The estimated fair value of options granted is determined as of the date of grant using the Black-Scholes option pricing model. The resulting fair value is recognized ratably over the requisite service period, which is generally the vesting period of the awards.

Share-based compensation expense for options granted under the 2011 Plan and the 2016 Plan is reflected in the statements of operations as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	391	$	290	$	1,182	$	882
General and administrative		525		384		1,461		1,176
Total	$	916	$	674	$	2,643	$	2,058

The following table summarizes the activity related to stock options during the nine months ended September 30, 2021:


				Weighted
	Number of			Average
	Shares			Exercise Price
Options outstanding at December 31, 2020		4,248,193		$	3.95
Granted		1,794,906			3.93
Exercised		(184,360	)		1.19
Forfeited		—			—
Options outstanding at September 30, 2021		5,858,739			4.03

Options exercisable at December 31, 2020		2,355,900			4.96

Options exercisable at September 30, 2021		3,081,524			4.58

As of September 30, 2021, the Company had $6.5 million of unrecognized compensation expense related to unvested stock options, which is expected to be recognized over a weighted average period of 2.7 years.

The Company has granted restricted stock units (“RSUs”) to employees from the 2016 Plan. The shares underlying the RSU awards have vesting terms of two to four years from the date of grant subject to the employees’ continuous service and subject to accelerated vesting in specified circumstances. The fair value of the RSUs granted is measured based on the market value of the Company’s common stock on the date of grant and is recognized ratably over the requisite service period, which is generally the vesting period of the awards.

The total share-based compensation expense related to RSUs is reflected in the statements of operations as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	194	$	70	$	561	$	304
General and administrative		190		80		552		348
Total	$	384	$	150	$	1,113	$	652

The following table summarizes the activity related to RSUs during the nine months ended September 30, 2021:


				Weighted Average
	Number of			Grant Date
	Shares			Fair Value
Non-vested RSUs outstanding at December 31, 2020		767,271		$	1.78
Granted		965,344			4.01
Vested		(415,268	)		1.27
Non-vested RSUs outstanding at September 30, 2021		1,317,347			3.58

As of September 30, 2021, the Company had $3.8 million of unrecognized compensation expense related to the RSUs which is expected to be recognized over a weighted average period of 3.1 years.

The 2016 Employee Stock Purchase Plan (the “2016 ESPP”) became effective on June 1, 2016. The 2016 ESPP is considered a compensatory plan and the fair value of the discount and the look-back period are estimated using the Black-Scholes option pricing model and expense is recognized over the six-month withholding period prior to the purchase date.

The share-based compensation expense recognized for the 2016 ESPP is reflected in the statements of operations and comprehensive loss as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	12	$	10	$	29	$	12
General and administrative		4		7		16		11
Total	$	16	$	17	$	45	$	23