In January 2023, our board of directors (the “Board”) approved the 2022 Employment Inducement Award Plan (the “2022 Inducement Plan”) which provides for the grant of non-qualified stock options, restricted stock awards (“RSAs”), restricted stock units (“RSUs”), performance awards, other stock awards and dividend equivalents to eligible employees with respect to an aggregate of up to 2,000,000 shares of our common stock. In June 2023, the 2022 Inducement Plan was terminated upon the approval of an amendment to the Company's 2019 Incentive Award Plan (the “2019 Plan”) at the annual meeting of the Company's stockholders (the “Annual Meeting”) in June 2023.

In 2019, the Board and stockholders approved the 2019 Plan. The 2019 Plan superseded and replaced in its entirety our 2010 Equity Incentive Plan (the “2010 Plan”) which was effective in March 2010, and no further awards will be granted under the 2010 Plan; however, the terms and conditions of the 2010 Plan will continue to govern any outstanding awards thereunder. The 2010 Plan provided for the grant of incentive stock options, non-statutory stock options, RSUs, RSAs, PSUs, PBOs, stock appreciation rights, and stock purchase rights to our employees, non-employee directors and consultants. The 2019 Plan provides for the grant of stock options, including incentive stock options and non-qualified stock options, stock appreciation rights, RSA, RSUs, performance-contingent restricted stock units (“PSUs”), performance based options (“PBOs”), other stock or cash based awards and dividend equivalents to eligible employees and consultants of the Company or any parent or subsidiary, as well as members of the Board.

The number of shares of our common stock that were initially available for issuance under the 2019 Plan is equal to the sum of (i) 7,897,144 shares, and (ii) any shares subject to awards granted under the 2010 Plan that were outstanding as of April 22, 2019 and thereafter terminate, expire, lapse or are forfeited. In June 2019, 8.1 million shares authorized for issuance under the 2019 Plan were registered under the Securities Act of 1933, as amended (the “Securities Act”). In April 2023, the Board approved an amendment to the 2019 Plan (the “2019 Amended Plan”) which became effective upon stockholders' approval at the Annual Meeting in June 2023. The 2019 Amended Plan included the (i) increase in the number of shares available by 8,000,000 shares, such that an aggregate of 15,897,144 shares are reserved for issuance under the 2019 Amended Plan and any shares subject to awards granted under the 2010 Plan, and (ii) increase in the number of shares that may be granted as incentive stock options under the 2019 Amended Plan such that an aggregate of 22,000,000 shares of common stock may be granted as incentive stock options under the 2019 Amended Plan.

In April 2023, the Board approved an employee stock purchase plan (the “ESPP”) which became effective upon approval at the Annual Meeting in June 2023. The ESPP allows eligible employees of the Company to purchase shares of our common stock through payroll deductions over 24-month offering periods. The per share purchase price will be the lower of 85% of the closing trading price per share of our common stock on the first trading date of an offering period in which a participant is enrolled or 85% of the closing trading price per share on the purchase date. Participant purchases are limited to a maximum of $25,000 of fair value of our stock per calendar year. The Company is authorized to grant up to 2,000,000 shares of common stock under the ESPP. The first offering period of the ESPP commenced in December 2023 and as of December 31, 2023, the Company had not issued any shares of common stock under the ESPP. We recognized $25 thousand of stock-based compensation expenses related to the ESPP in the year ended December 31, 2023.

The option exercise price for incentive stock options must be at least 100% of the fair value of our common stock on the date of grant and the option exercise price for non-statutory stock options is at least 85% of the fair value of our common stock on the date of grant, as determined by the Board. If, at the time of a grant, the optionee directly or by attribution owns stock possessing more than 10% of the total combined voting power of all of our outstanding capital stock, the exercise price for these options must be at least 110% of the fair value of the underlying common stock. Stock options granted to employees generally have a maximum term of ten years and vest over four years from the date of grant, of which 25% vest at the end of one year, and 75% vest monthly over the remaining three years. We may grant options with different vesting terms from time to time. Unless an employee's termination of service is due to disability or death, upon termination of service, any unexercised vested options will be forfeited at the end of three months or the expiration of the option, whichever is earlier.

We also grant employees RSUs, which generally vest over either a three year period with 33% of the shares subject to the RSUs vesting on each yearly anniversary of the vesting commencement date or over a four-year period with 25% of the shares subject to the RSU vesting on each yearly anniversary of the vesting commencement date, in each case contingent upon such employee’s continued service on such vesting date. RSUs are generally subject to forfeiture if employment terminates prior to the release of vesting restrictions. We may grant RSUs with different vesting terms from time to time.

In prior years, the compensation committee of the Board approved grants of PBOs and PSUs to our executives, and solely in respect of non-executive employees, delegated to our CEO the authority to approve grants of PSUs. The PSUs and PBOs vest based upon both the successful achievement of certain corporate operating milestones in specified timelines and continued employment through the applicable vesting date. When the performance goals are deemed to be probable of achievement for these types of awards, recognition of stock-based compensation expense commences. Once the number of shares eligible to vest is determined, those shares vest in two equal installments with 50% vesting upon achievement, as determined by the compensation committee of the Board, and the remaining 50% vesting on the first anniversary of achievement, in each case, subject to the recipient’s continued service through the applicable vesting date. If the performance goals are achieved at the threshold level, the number of shares eligible to vest in respect of the PSUs and PBOs would be equal to half the number of PSUs granted and one-quarter the number of shares underlying the PBOs granted. If the performance goals are achieved at the target level, the number of shares eligible to vest in respect of the PSUs and PBOs would be equal to the number of PSUs granted and half of the shares underlying the PBOs granted. If the performance goals are achieved at the superior level, the number of shares eligible to vest in respect of the PSUs would be equal to two times the number of PSUs granted and equal to the number of PBOs granted. The number of shares issuable upon achievement of the performance goals at the levels between the threshold and target levels for the PSUs and PBOs or between the target level and superior levels for the PSUs would be determined using linear interpolation. Achievement below the threshold level would result in no shares being eligible to vest in respect of the PSUs and PBOs.

No PSUs and PBOs were granted in 2023. In 2022, we awarded PSUs (“2022 PSUs”) and PBOs (“2022 PBOs”), each of which commence vesting based upon the achievement of various weighted performance goals, including finance and corporate strategy, performance enzymes and biotherapeutics deliverables, research plans, and organizational development. In the first quarter of 2023, the compensation committee of the Board determined that the 2022 PSUs and 2022 PBOs performance goals had been achieved at 85.0% and 42.5% of the target level, respectively, and recognized stock-based compensation expenses accordingly. Accordingly, 50% of the shares underlying the 2022 PSUs and PBOs vested in the first quarter of 2023 and 50% of the shares underlying the 2022 PSUs and PBOs will vest in the first quarter of 2024, in each case, subject to the recipient’s continued service on each vesting date.

In 2021, we awarded PSUs (“2021 PBOs”) and PBOs (“2021 PBOs”), each of which commence vesting based upon the determination of the compensation committee of the Board of the achievement of various weighted performance goals, including total revenues, product revenue, performance enzymes pipeline advancements, biotherapeutics pipeline advancements, organization and infrastructure upgrades, and significant events that can be publicly announced. In the first quarter of 2022, we determined that the 2021 PSUs and 2021 PBOs performance goals had been achieved at 146% and 73% of the target level, respectively, and recognized stock-based compensation expenses accordingly. Accordingly, 50% of the shares underlying the 2021 PSUs and PBOs vested in the first quarter of 2022 and 50% of the shares underlying the 2021 PSUs and PBOs vested in the first quarter of 2023, in each case, subject to the recipient’s continued service on each vesting date.

In connection with the retirement of John Nicols, our former President and Chief Executive Officer, in August 2022, and the Transition and Separation Agreement between Mr. Nicols and the Company, certain supplementary modifications were made to Mr. Nicols' vested and unvested stock option and PBOs awards including voluntary forfeiture of certain unvested stock option and PBOs awards and the extension of the post-termination exercise period of certain vested stock option and PBOs awards. During the year ended December 31, 2022, we recorded a one-time, non-cash incremental compensation expense of $1.0 million, net of the required reversal of previously recognized stock-based compensation expenses attributed to unvested shares, in selling, general and administrative expenses related to these stock option award modifications.

We estimated the fair value of stock options using the Black-Scholes-Merton option-pricing model based on the date of grant. The following summarizes the weighted-average assumptions used to estimate the fair value of employee stock options granted:


	Year Ended December 31,
	2023		2022		2021
Expected life (years)	5.8		5.7		5.6
Volatility	66.2	%	62.1	%	52.5	%
Risk-free interest rate	4.0	%	3.1	%	0.8	%
Expected dividend yield	0.0	%	0.0	%	0.0	%

The following summarizes the weighted-average assumptions used to estimate the fair value of 50,000, nil, and 9,000 shares of stock options granted to non-employees for services valued at $0.1 million, nil, and $0.1 million during the years ended December 31, 2023, 2022, and 2021 respectively:


	Year Ended December 31,
	2023		2022		2021
Expected life (years)	5.8		0.0		5.6
Volatility	70.1	%	—	%	54.1	%
Risk-free interest rate	4.7	%	—	%	0.9	%
Expected dividend yield	0.0	%	0.0	%	0.0	%

The following tables summarizes stock option activities:


	Number of Shares	Weighted Average Exercise Price Per Share
	(In Thousands)
Outstanding at December 31, 2020	3,385	$	7.19
Granted	286	$	26.85
Exercised	(664)	$	6.96
Forfeited/Expired	(72)	$	17.99
Outstanding at December 31, 2021	2,935	$	8.90
Granted	2,000	$	8.90
Exercised	(410)	$	2.33
Forfeited/Expired	(275)	$	19.01
Outstanding at December 31, 2022	4,250	$	8.88
Granted	2,046	$	5.23
Exercised	(283)	$	1.97
Forfeited/Expired	(839)	$	12.08
Outstanding at December 31, 2023	5,174	$	7.31

The following table summarizes RSA activities:


	Number of Shares	Weighted Average Grant Date Fair Value Per Share
	(In Thousands)
Non-vested balance at December 31, 2020	96	$	11.44
Granted	46	$	21.91
Vested	(62)	$	11.31
Non-vested balance at December 31, 2021	80	$	17.53
Granted	159	$	7.53
Vested	(58)	$	18.42
Non-vested balance at December 31, 2022	181	$	8.45
Granted	277	$	2.89
Vested	(133)	$	9.04
Non-vested balance at December 31, 2023	325	$	3.48

The following table summarizes RSU activities:


	Number of Shares	Weighted Average Grant Date Fair Value Per Share
	(In Thousands)
Non-vested balance at December 31, 2020	176	$	14.17
Granted	163	$	26.59
Vested	(70)	$	13.57
Forfeited/Expired	(37)	$	21.89
Non-vested balance at December 31, 2021	232	$	21.83
Granted	518	$	17.46
Vested	(106)	$	21.21
Forfeited/Expired	(126)	$	19.55
Non-vested balance at December 31, 2022	518	$	18.15
Granted	1,049	$	5.24
Vested	(204)	$	18.42
Forfeited/Expired	(343)	$	9.71
Non-vested balance at December 31, 2023	1,020	$	7.66

The following table summarizes PSU activities:


	Number of Shares	Weighted Average Grant Date Fair Value Per Share
	(In Thousands)
Non-vested balance at December 31, 2020	131	$	15.34
Granted	82	$	26.16
Vested	(66)	$	16.14
Forfeited/Expired	(19)	$	19.38
Non-vested balance at December 31, 2021	128	$	21.24
Granted	686	$	9.55
Vested	(107)	$	20.52
Forfeited/Expired	(40)	$	19.93
Non-vested balance at December 31, 2022	667	$	9.41
Vested	(315)	$	11.02
Forfeited/Expired	(75)	$	12.49
Other	15	$	25.05
Non-vested balance at December 31, 2023	292	$	7.69

We estimated the fair value of PBOs using the Black-Scholes-Merton option-pricing model based on the date of grant. No PBOs were granted to employees for their services during the year ended December 31, 2023. The following summarize the weighted-average assumptions used to estimate the fair value of PBOs granted:


	Year Ended December 31,
	2022		2021
Expected life (years)	5.6		5.5
Volatility	54.9	%	51.9	%
Risk-free interest rate	1.8	%	0.7	%
Expected dividend yield	0.0	%	0.0	%

The following tables summarizes PBOs activities:


	Number of Shares	Weighted Average Grant Date Fair Value Per Share
	(In Thousands)
Outstanding at December 31, 2020	1,560	$	5.05
Granted	433	$	12.23
Exercised	(35)	$	9.02
Forfeited/Expired	(118)	$	12.23
Outstanding at December 31, 2021	1,840	$	4.11
Granted	733	$	9.89
Forfeited/Expired	(747)	$	8.29
Outstanding at December 31, 2022	1,826	$	4.70
Forfeited/Expired	(178)	$	9.73
Outstanding at December 31, 2023	1,648	$	5.64

The fair value of shares to be issued under the ESPP is computed using the Black-Scholes-Merton option pricing model at the commencement of the offering period. The following summarizes the weighted-average assumptions used to estimate the fair value of ESPP for the initial offering period:


	Year Ended December 31,
	2023
Expected life (years)	0.4
Volatility	89.6	%
Risk-free interest rate	5.3	%
Expected dividend yield	0.0	%