Stock option activity under the 2019 Equity Incentive Plan (the “2019 Plan”) is set forth below for the nine months ended September 30, 2021:

				Weighted		Average
				Average		Remaining		Aggregate
	Number of			Exercise		Contractual		Intrinsic
	Shares			Price Per		Term		Value
	Outstanding			Share		(Years)		(in thousands)
Balances at December 31, 2020		1,481,100		$	16.13		8.36	$	11,123
Stock options authorized		—
Stock options granted		726,195			20.56
Stock options exercised		(104,104	)		10.19
Stock options forfeited		(120,893	)		21.07
Balances at September 30, 2021		1,982,298		$	17.77	8.22		$	28,364

As of September 30, 2021, 2,746,829 shares remained available for issuance under the 2019 Plan.

Restricted stock unit (“RSU”) activity under the 2019 Plan is set forth below for the nine months ended September 30, 2021:

				Weighted
				Average
	Number of			Grant Date
	Shares			Fair Value
	Outstanding			Per Share
Balances at December 31, 2020		56,500		$	44.66
RSUs granted		—			—
RSUs vested and settled		(14,125	)		44.66
RSUs forfeited		(1,875	)		44.66
Balances at September 30, 2021		40,500		$	44.66

Total stock-based compensation expense recognized for options and RSUs granted to both employees and non-employees and for the employee stock purchase plan was as follows (in thousands):

	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	1,207	$	1,125	$	4,308	$	3,450
General and administrative		1,084		880		3,558		2,678
Total stock-based compensation expense	$	2,291	$	2,005	$	7,866	$	6,128

As of September 30, 2021, unrecognized stock-based compensation expense related to outstanding unvested stock options and RSUs that are expected to vest was $16.1 million. This unrecognized stock-based compensation expense is expected to be recognized over 2.7 years.

In October 2019, the Company adopted the 2019 Employee Stock Purchase Plan (the “2019 ESPP”). The Company reserved 240,336 shares of common stock pursuant to purchase rights to be granted to the Company’s employees. The 2019 ESPP provides that the number of shares reserved and available for issuance will automatically increase on January 1 of each calendar year, beginning January 1, 2020, by the lesser of (1) 1.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, (2) 240,336 shares or (3) a number determined by the board of directors that is less than (1) and (2).

Under the 2019 ESPP, eligible employees are granted rights to purchase shares of common stock, which can be funded through payroll deductions that cannot exceed 15% of each employee’s compensation. The 2019 ESPP generally provides for a 24-month offering period, which includes four six-month purchase periods. At the end of each purchase period, eligible employees are permitted to purchase shares of common stock at 85% of the lower of fair market value at the beginning of the offering period or fair market value at the end of the purchase period. The 2019 ESPP is considered a compensatory plan and the Company recorded stock-based compensation expense of $0.5 million and $2.3 million for the three and nine months ended September 30, 2021, respectively, and $0.7 million and $2.0 million for the three and nine months ended September 30, 2020, respectively.