A summary of stock option activity for the six months ended June 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		(1,911,472	)		1,911,472		$	15.04
Options exercised		—			(433,594	)	$	2.91
Options forfeited or cancelled		745,006			(745,006	)	$	14.26
Outstanding, June 30, 2021		2,860,244			4,439,817		$	12.90		8.20	$	4,948
Shares exercisable, June 30, 2021					1,288,596		$	8.43		5.20	$	4,522
Vested and expected to vest, June 30, 2021					4,439,817		$	12.90		8.20	$	4,948

In addition, the Company granted 6,410 shares of a performance restricted stock unit (PSU) in second quarter of 2021.

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


	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development	$	825	$	88	$	2,413	$	174
General and administrative		1,834		263		3,289		476
Total stock-based compensation	$	2,659	$	351	$	5,702	$	650

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


	Three Months Ended June 30,		Six Months Ended June 30,
	2021	2020	2021	2020
Fair value of common stock	$10.29 - $15.93	N/A	$10.29 - $16.82	$1.48
Expected term (years)	0.9 - 6.1	N/A	0.9 - 6.1	6.1 - 6.8
Volatility	77.9% - 78.6%	N/A	77.9% - 79.8%	80.3% - 82.6%
Risk free rates	0.1% - 1.1%	N/A	0.1% - 1.1%	0.4% - 0.5%
Dividend rate	0.0%	N/A	0.0%	0.0%

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 June 30, 2021, the number of shares of common stock available for grant under the 2017 and 2018 Plan is 2,529,290 shares. As of June 30, 2021, an aggregate of 2,506,532 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 $15.40 per share. Included in this amount was a grant of 6,410 shares related to a PSU the Company granted in May 2021. This PSU provides for 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 June 30, 2021, the number of shares of common stock available for grant under the 2014 and 2015 Plans is 324,543. As of June 30, 2021, an aggregate of 1,878,756 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 $8.71 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 six months ended June 30, 2021.

As of June 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.