In 2013, Twist Holdings, LLC and Advance Holdings, LLC adopted equity incentive plans, which we refer to collectively as the 2013 Plan, pursuant to which the board of managers could grant options to purchase Class A units to officers and employees. Options could be granted with an exercise price equal to or greater than the unit’s fair value at the date of grant. All issued awards have 10-year terms and generally vest and become fully exercisable annually over five years of service from the date of grant. Awards will become fully vested upon the sale of the Company. In March 2018, the 2013 Plan was amended to increase the maximum number of Class A units to 6,207,978.

In September 2018, the board of directors adopted the 2019 Equity Incentive Plan, or the 2019 Plan, which became effective in June 2019. Under the 2019 Plan, a total of 4,500,000 shares of our Class A common stock were reserved for issuance as options, stock appreciation rights, restricted stock, restricted stock units, or RSUs, performance units or performance shares. Upon the completion of our IPO, the 2019 Plan replaced the 2013 Plan; however, the 2013 Plan will continue to govern the terms and conditions of the outstanding awards previously granted under that plan. The number of shares that will be available for issuance under our 2019 Plan also will increase annually on the first day of each year beginning in 2020, in an amount equal to the least of: (a) 6,900,000 shares, (b) 5% of the outstanding shares of all classes of our common stock as of the last day of the immediately preceding year and (c) such other amount as our board of directors may determine. All future grants going forward will be issued under the 2019 Plan. On January 1, 2021, the number of shares available under the 2019 Plan was increased by 2.0 million shares. As of September 30, 2021, approximately 4.9 million shares of Class A common stock remain available for future issuance under the 2019 Plan.

Option activity for the nine months ended September 30, 2021 under the 2013 and 2019 Plans is as follows:


	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value (000's)
Balance at January 1, 2021		4,121,225		$	9.20		7.0	$	87,842
Granted		140,569			51.16		9.5
Exercised		(1,448,061	)		6.53		—
Forfeited		(42,878	)		15.69		—
Expired		(2,816	)		15.62		—
Balance at September 30, 2021		2,768,039			12.62		7.5		136,212
Exercisable at September 30, 2021		813,278			10.00		6.0		42,105
Vested and expected to vest		2,768,039			12.62		7.5		136,212

RSU award activity for the nine months ended September 30, 2021 under the 2019 Plan is as follows:


	Class A Common Stock			Weighted Average Grant Date Fair Value		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value (000's)
Unvested at January 1, 2021		20,140		$	15.89		1.2	$	628
Granted (1)		28,224			48.16		0.5
Released		(12,742	)		15.70
Forfeited		—			—
Unvested at September 30, 2021		35,622			41.53		0.7		2,200

(1)

Includes an adjustment of 11,763 shares underlying performance-based RSU awards made during the nine months ended September 30, 2021. The vesting of such RSUs is based upon the Company’s current performance against predefined financial targets.

There were 28,250 options and no RSUs granted during the three months ended September 30, 2021 and 140,569 options and 28,224 RSUs granted during the nine months ended September 30, 2021. The weighted average grant-date fair value of options granted during the three and nine months ended September 30, 2021 was $29.13 per share and $21.15 per share, respectively. The weighted average grant-date fair value of RSUs granted during the nine months ended September 30, 2021 was $48.16 per share.

As of September 30, 2021, there was $11.9 million of total unrecognized compensation cost related to unvested options and RSUs granted under the 2013 Plan and 2019 Plan, which is expected to be recognized over a weighted average service period of 3.2 years.

Equity‑based compensation cost that has been included in general and administrative expense in the accompanying condensed consolidated statements of income amounted to $1.4 million and $1.0 million for the three months ended September 30, 2021 and 2020, respectively, and $3.7 million and $2.4 million for the nine months ended September 30, 2021 and 2020, respectively. There was an excess income tax benefit of $2.2 million and $3.4 million recognized in the condensed consolidated statements of income for equity‑based compensation arrangements for the three months ended September 30, 2021 and 2020, respectively, and $15.7 million and $5.1 million for the nine months ended September 30, 2021 and 2020, respectively.