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


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	796	$	951	$	3,209	$	1,125
General and administrative		1,733		2,015		5,022		2,491
Total stock-based compensation	$	2,529	$	2,966	$	8,231	$	3,616

A summary of stock option activity for the nine months ended September 30, 2021 is set forth below:


	Number of Shares Available for Grant			Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2020		1,739,621			3,706,945		$	10.90		7.98	$	15,126
Options authorized		2,287,089			—
Options granted		(2,171,632	)		1,954,472		$	14.89
Options exercised		—			(546,642	)	$	2.98
Options forfeited or cancelled		838,419			(838,419	)	$	14.15
Outstanding, September 30, 2021		2,693,497			4,276,356		$	13.10		7.96	$	2,101
Options exercisable, September 30, 2021					1,329,752		$	9.77		5.33	$	1,907
Vested and expected to vest, September 30, 2021					4,276,356		$	13.10		7.96	$	2,101

The assumptions used in the Black Scholes Model to calculate stock-based compensation are as follows:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2021	2020	2021	2020
Fair value of common stock	$7.84 - $8.35	$5.96 - $10.66	$7.84 - $16.82	$4.85 - $10.66
Expected term (years)	5.6 - 6.1	1.0 - 6.1	0.9 - 6.1	1.0 - 6.1
Volatility	78.0% - 78.2%	77.1% - 96.4%	77.9% - 79.8%	72.6% - 96.3%
Risk free rates	0.9% - 1.2%	0.1% - 0.4%	0.1% - 1.2%	0.1% - 1.7%
Dividend rate	0.0%	0.0%	0.0%	0.0%

In May 2021, the Company granted 6,410 performance stock units (PSUs) to an employee with a grant date fair value of $15.60 per share. The PSUs provide immediate acceleration of vesting in the event of a certain performance milestone to be achieved by March 31, 2022. The probability of achieving this milestone is expected to be 100% as of September 30, 2021. In addition, the Company granted 210,750 restricted stock units (RSUs) to employees in August 2021 with a weighted-average grant date fair value of $7.12 per share.

The summary of RSU activity and related information follows:


	Number of Units Outstanding		Weighted- Average Grant Date Fair Value
Outstanding, December 31, 2020		—
RSUs granted		217,160	$	7.37
RSUs Vested		—
RSUs forfeited		—
Outstanding, September 30, 2021		217,160	$	7.37

The Company has 2014 Share Option Plan (the 2014 Plan), 2015 Stock Incentive Plan (the 2015 Plan), 2017 Stock Incentive Plan (the 2017 Plan), 2018 Stock Incentive Plan (the 2018 Plan), and 2018 Employee Stock Purchase Plan (the 2018 ESPP, and, collectively with the 2014 Plan, the 2015 Plan, the 2017 Plan and the 2018 Plan, the Plans). There have been no material changes in the Plans from those disclosed in Note 18 to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020.

As of September 30, 2021, the number of shares of common stock available for grant under the 2017 and 2018 Plan is 2,349,535 shares. As of September 30, 2021, an aggregate of 2,686,287 shares of common stock were issuable upon the exercise of outstanding stock options under the 2017 Plan and 2018 Plans at a weighted average exercise price of $15.41 per share. Included in this amount was a grant of 6,410 PSUs and 210,750 shares of RSUs the Company granted in May 2021 and August 2021, respectively.

As of September 30, 2021, the number of shares of common stock available for grant under the 2014 and 2015 Plans is 343,961. As of September 30, 2021, an aggregate of 1,421,746 shares of Former Adicet common stock were issuable upon the exercise of outstanding stock options under the 2015 plan at a weighted average exercise price of $8.96 per share and an aggregate of 22,989 shares of Former Adicet common stock were issuable upon the exercise of outstanding stock options under the 2014 Plan at a weighted average exercise price of $1.61 per share.

On January 1, 2021, as a result of the foregoing evergreen provision, the number of shares of common stock available for issuance under the 2018 ESPP automatically increased from 131,432 to 209,135 shares. During the 2021 Annual Meeting of the stockholders held on April 27, 2021, the stockholders approved an amendment and restatement of the Company's 2018 ESPP. As a result, the Company increased the shares available for issuance under the 2018 ESPP to 524,775 shares. No shares have been issued under the 2018 ESPP during the three and nine months ended September 30, 2021.

As of September 30, 2021, an aggregate of 362,503 shares of were issuable upon the exercise of inducement grants of stock options approved by the Company in accordance with Nasdaq listing Rule 5635(c)(4) at a weighted average exercise price of $14.33 per share.