The Company’s equity incentive plans, the 2018 Long Term Incentive Plan, as amended (the “2018 LTIP”), 2020 Employment Inducement Incentive Plan, as amended (the “2020 EIIP”), and previously, the Amended and Restated 2012 Long Term Incentive Plan (the “2012 LTIP”), reserve ordinary shares for the issuance of stock options, stock appreciation rights, restricted shares, RSUs, performance bonus awards, performance share units awards, dividend equivalents and other share or cash-based awards to eligible individuals. Options granted under each of the 2018 LTIP, 2020 EIIP, and 2012 LTIP expire no later than ten years from the date of grant.

In May 2023, the Company’s shareholders approved an amendment to the 2018 LTIP to increase the number of ordinary shares available for issuance under the 2018 LTIP by 2,000,000 ordinary shares. As of December 31, 2023, the number of ordinary shares authorized under the 2018 LTIP was 14,614,183. Upon adoption of the 2018 LTIP, no new awards are permitted under the 2012 LTIP.

As of December 31, 2023, the number of ordinary shares authorized under the 2020 EIIP was 1,485,000 and 58,750 ordinary shares remained available for future awards under the 2020 EIIP. The Company’s Board of Directors has adopted a series of amendments to increase the ordinary shares available for issuance under the 2020 EIIP and it reserves the right to both amend the 2020 EIIP to increase the number of ordinary shares available and make additional awards to key new hires.

The Company’s option awards generally vest over four years, while RSUs vest over two years. As of December 31, 2023, 3,585,452 ordinary shares remained available for grant under its equity plans.

Share-based compensation expense recorded in these Consolidated Financial Statements for the years ended December 31, 2023, 2022 and 2021, respectively, was based on awards granted under the 2012 LTIP, the 2018 LTIP, and the 2020 EIIP. The estimated forfeiture rate as of December 31, 2023 was 8%. Changes in the estimates and assumptions relating to forfeitures may cause the Company to realize material changes in share-based compensation expense in the future.

The amount of unearned share-based compensation related to unvested stock options at December 31, 2023, is $89.4 million. The weighted-average period over which this unearned share-based compensation is expected to be recognized is 2.43 years.


	Year Ended December 31,
	2023		2022		2021
Research and development	$	19,211	$	14,805	$	9,514
General and administrative	21,703		16,517		15,144
Total share-based compensation expense	$	40,914	$	31,322	$	24,658

The Company recognized tax benefits from share-based awards of $7.2 million, $5.8 million, and $4.7 million, for the years ended December 31, 2023, 2022 and 2021, respectively.

With the exception of the options granted on February 12, 2021 pursuant to an option exchange program, where no incremental share-based compensation expense was recognized, the fair value of the options granted to employees and non-employee directors during the years ended December 31, 2023, 2022 and 2021, respectively was estimated as of the grant date using the Black-Scholes option-pricing model assuming the weighted-average assumptions listed in the following table:


	Year Ended December 31,
	2023		2022		2021
Expected volatility	81.2	%	82.4	%	81.7	%
Risk-free interest rate	4.2	%	2.2	%	1.0	%
Expected dividend yield	—	%	—	%	—	%
Expected life (in years)	4.8		6.0		6.0
Weighted average grant date fair value	$37.32		$23.43		$21.39

The fair value of employee stock options is amortized on a straight-line basis over the requisite service period for each award. Each of the inputs discussed above is subjective and generally requires significant management judgment to determine. The simplified method was used to estimate the expected term of all options in previous years given lack of historical experience. Beginning January 1, 2023, expected term is estimated based on historical experience.


	Options	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2022	9,479,998	$	23.16	6.82	$	354,856
Granted	1,790,621	56.48
Exercised	(1,135,302)	18.90
Forfeited	(246,064)	44.28
Expired	(22,916)	70.74
Outstanding at December 31, 2023	9,866,337	$	29.06	6.60	$	118,447
Vested and expected to vest at December 31, 2023	9,558,866	$	28.49	6.53	$	117,705
Vested at December 31, 2023	6,371,511	$	20.75	5.54	$	105,546

The total intrinsic value of options exercised was $52.1 million, $49.2 million, and $33.9 million during the years ended December 31, 2023, 2022 and 2021, respectively, determined as of the date of exercise.


	Number of Units	Weighted Average Grant-Date Fair Value		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Unvested at December 31, 2022	23,000	$	60.89	1.67	$	1,386
Units Granted	8,000	51.80
Units Vested	(5,750)	60.89
Units Forfeited	—	—
Unvested at December 31, 2023	25,250	$	58.01	1.09	$	918
Unvested and expected to vest at December 31, 2023	23,066	$	58.19	1.08	$	838

The fair value of RSUs was determined on the date of grant based on the market price of the Company’s ordinary shares as of that date. The fair value of the RSUs is recognized as an expense on a straight-line basis over the vesting period of each RSU. Upon the vesting of the RSUs, a portion of the shares vested are sold by the employee to satisfy employee withholding tax requirements (sell-to-cover). The total fair value of shares vested during the year ended December 31, 2023 was $0.2 million. As of December 31, 2023, total compensation cost not yet recognized related to unvested RSUs was $1.0 million, which is expected to be recognized over a weighted-average period of 1.17 years. RSUs settle into ordinary shares upon vesting.