Note 15. Equity Incentive Plans

In February 2005, the Company adopted the 2005 Stock Plan, as amended in January 2010 and November 2012 (the “2005 Plan”). Under the 2005 Plan provisions, the Company was authorized to grant incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock units ("RSUs"), and shares of restricted stock.

In May 2017, the Board and the Company’s stockholders approved the 2017 Equity Incentive Plan (the “2017 Plan”). As a result of the adoption of the 2017 Plan, no further grants may be made under the 2005 Plan. The 2017 Plan provides for the issuance of stock options, RSUs and other awards to employees, directors and consultants of the Company. The 2017 Plan includes an evergreen provision that provides for the number of shares of common stock reserved for issuance under the 2017 Plan to automatically increase on January 1 of each year by the lesser of (1) 5% of the number of shares of the Company’s common stock outstanding on December 31st of the preceding calendar year or (2) such number of shares as determined by the board of directors.

Stock Options

Incentive stock options may only be granted to Company employees and may only be granted with an exercise price not less than the fair value of the common stock, or not less than 110% of fair value when the grant is issued to a person who, at the time of grant, owns stock representing more than 10% of the voting power of all classes of stock. Non-statutory stock options may be granted to Company employees, directors and consultants, and may be granted at a price per share not less than fair value on the date of the grant.

Options granted under the 2005 Plan and 2017 Plan generally vest over four years and expire no later than 10 years from the grant date. The 2005 Plan and 2017 Plan grants the board of directors discretion to determine when the options granted will become exercisable.

Compensation expense for stock options is based upon the estimated fair value of the awards. The fair value of stock option grants is determined using the Black-Scholes option pricing model which requires the use of certain assumed inputs. The assumed inputs used to determine the fair value of stock options granted for the years ended December 31, 2022, 2021, and 2020 are set forth below:

At December 31, 2022, total unrecognized stock-based compensation cost related to unvested stock options was $10.5 million, which will be recognized ratably over a weighted-average period of 2.8 years.

No income tax benefits from stock-based compensation arrangements have been recognized in the consolidated statements of operations.

Restricted Stock Units

The Company grants RSUs under the 2017 Plan to executive management and its non-employee directors. RSUs granted to executive management generally vest over four years, while RSUs granted to non-employee directors generally vest annually. A new non-employee director will receive an initial grant upon joining the board of directors and all non-employee directors receive new annual grants at each annual meeting of stockholders. Compensation expense for RSUs is based upon the estimated fair value of the awards on the date of grant.

At December 31, 2022, total unrecognized stock-based compensation cost related to RSUs was $5.7 million, which will be recognized ratably over a weighted-average period of 2.6 years.

During the year ended December 31, 2022, the Company modified RSUs to accelerate vesting for two individuals in respect of 5,849 RSUs and cancelled the award of another individual in respect of 1,887 RSUs. The Company accounted for these as modifications of those awards and recognized net incremental compensation expense of $0.01 million.

The incremental compensation cost is measured as the excess of the fair value of the modified award over the fair value of the original award immediately before its terms were modified and recognized as compensation expense on the date of modification for vested awards

2017 Employee Stock Purchase Plan

In May 2017, the Board and the Company’s stockholders adopted the 2017 Employee Stock Purchase Plan (“2017 ESPP”).The 2017 ESPP permits the maximum discounted purchase price permitted under U.S. tax rules, including a “lookback”, which allows eligible employees to purchase shares of the Company’s common stock at a 15% discount to the lesser of the fair market value of common stock at the beginning and end of the offering period.

The 2017 ESPP initial offering period, which began in June 2017, ran for approximately 24 months in length, and contained four 6-month purchase periods. Subsequent offering periods generally run for six months each. An employee’s purchase rights terminate immediately upon termination of employment or other withdrawal from the 2017 ESPP. No participant will have the right to purchase shares of common stock in an amount that has a fair market value of more than $25,000 determined as of the first day of the applicable purchase period, for each calendar year.

The 2017 ESPP contains a provision which provides for an automatic annual share increase on January 1 of each year, in an amount equal to the lesser of (1) 2% of the total number of shares of common stock outstanding on December 31st of the preceding calendar year, (2) 150,000 shares or (3) such number of shares as determined by the board of directors. The Company's board of directors authorized the automatic increase to the 2017 ESPP plan on January 1, 2022 for the year ended December 31, 2022.

Stock-Based Compensation Expense