The Company’s 2017 Stock Option and Incentive Plan (the “2017 Stock Plan”) became effective in November 2017, upon the closing of the Company’s initial public offering and will expire in October 2027. Under the 2017 Stock Plan, the Company may grant incentive stock options, non-statutory stock options, restricted stock awards, restricted stock units (“RSUs”) and other stock-based awards. The Company’s 2014 Stock Incentive Plan (the “2014 Stock Plan”) was terminated in November 2017 effective upon the completion of the Company’s initial public offering and no further options will be granted under the 2014 Stock Plan. At March 31, 2022, there were 598,619 options outstanding under the 2014 Stock Plan.

As of March 31, 2022, there were 6,143,876 shares of the Company’s common stock authorized for issuance under the 2017 Stock Plan, including 359,652 options that have been forfeited from the 2014 Stock Plan.

At March 31, 2022, there were 3,228,289 options available for issuance under the 2017 Stock Plan, 2,843,533 options outstanding and 42,250 RSUs outstanding. Awards granted under the 2017 Stock Plan have a term of ten years. Vesting of awards under the 2017 Stock Plan is determined by the board of directors, but is generally over one to four-year terms.

The fair value of options at date of grant was estimated using the Black-Scholes option-pricing model with the following assumptions:


	Three Months Ended March 31,
	2021		2022
Risk-free interest rate	0.50% - 0.72%		1.67% - 1.90%
Expected dividend yield	0%		0%
Expected life	5.5-6.7 years		5.6-6.7 years
Expected volatility	72%-74%		70%-72%
Weighted-average grant date fair value	$	4.36	$	2.93

The following table summarizes information about stock option activity during the three months ended March 31, 2022 (in thousands, except share and per share data):


	NUMBER OF SHARES			WEIGHTED- AVERAGE EXERCISE PRICE		WEIGHTED- AVERAGE REMAINING CONTRACTUAL TERM		AGGREGATE INTRINSIC VALUE
Outstanding, December 31, 2021		2,662,752		$	6.28
Granted		795,119			4.61
Exercised		(4,781	)		4.42
Forfeited		(10,938	)		8.32
Outstanding, March 31, 2022		3,442,152		$	5.89		7.83	$	2,389
Vested and exercisable, March 31, 2022		1,582,865		$	6.18		6.33	$	1,356
Vested and expected to vest, March 31, 2022		2,777,959		$	5.95		7.52	$	1,997

The following table summarizes information about RSU activity during the three months ended March 31, 2022:


	RSUs		AVERAGE GRANT DATE FAIR VALUE (IN DOLLARS PER SHARE)
Outstanding, December 31, 2021		42,250	$	3.25
Granted		—		—
Vested		—		—
Forfeited		—		—
RSUs outstanding at March 31, 2022		42,250	$	3.25

Unrecognized compensation expense related to unvested options as of March 31, 2022 was $3.8 million and will be recognized over the remaining vesting periods of the underlying awards. The weighted-average period over which such compensation is expected to be recognized is 2.6 years. Unrecognized compensation expense related to unvested RSUs as of March 31, 2022 was $18,000 and will be recognized over the remaining vesting periods of the underlying awards. The weighted-average period over which such compensation is expected to be recognized is 6 months.

In October 2017, the board of directors approved the 2017 Employee Stock Purchase Plan ("the ESPP") which became effective in November 2017, upon the closing of the Company's IPO. As part of the ESPP, eligible employees may acquire an ownership interest in the Company by purchasing common stock, at a discount, through payroll deductions. Eligible employees who elected

to participate were able to participate in the ESPP beginning September 1, 2021. As of March 31, 2022, there were 1,188,629 shares of common stock available for issuance under the ESPP.

The Company recorded stock-based compensation expense in the following expense categories of its accompanying condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2021 and 2022 (in thousands):



	Three Months Ended March 31,
	2021		2022
Research and development	$	226	$	254
General and administrative		295		382
Total	$	521	$	636