Note 8. Stock-Based Compensation

Employee Stock Purchase Plan

The Company maintains an Employee Stock Purchase Plan (the “Purchase Plan”), which is intended to qualify as an employee stock purchase plan within the meaning of Section 423(b) of the Internal Revenue Code. Under the Purchase Plan, the Company’s Board of Directors may authorize participation by eligible employees, including officers, in periodic offerings. Under the Purchase Plan eligible employee participants may purchase shares of common stock of the Company at a purchase price equal to 85% of the lower of the fair market value per share on the start date of the offering period or the fair market value per share on the purchase date. The Purchase Plan consists of a fixed offering period of 12 months with two purchase periods within each offering period. At September 30, 2018, the Company had 0.8 million shares available for future issuance.

2008 Equity Incentive Plan and Inducement Plan

The Company also maintains an equity compensation plan to provide long-term incentives for employees, contractors, and members of its Board of Directors. The Company currently grants equity awards from one plan, the 2008 Equity Incentive Plan (the “2008 Plan”). The 2008 Plan allows for the issuance of non-statutory and incentive stock options, restricted stock, restricted stock units (“RSUs”), stock appreciation rights, other stock-related awards, and performance awards which may be settled in cash, stock, or other property. On June 6, 2012 and June 12, 2013, the stockholders approved amendments to the 2008 Plan (collectively the “Amended 2008 Plan”) such that the Amended 2008 Plan had reserved for issuance an amount not to exceed 19.5 million shares. On June 10, 2015, the Company’s stockholders approved an amendment and restatement of the 2008 Plan that increased the aggregate number of shares of common stock authorized for issuance under the 2008 Plan by 5,000,000 shares. On June 7, 2017, the Company’s stockholders approved an amendment and restatement of the 2008 Plan that increased the aggregate number of shares of common stock authorized for issuance under the 2008 Plan by 6,000,000 shares. Awards under the Amended 2008 Plan generally have a maximum term of 10 years from the date of the award. The Amended 2008 Plan generally requires options to be granted at 100% of the fair market value of the Company’s common stock subject to the option on the date of grant. Options granted by the Company to employees generally vest over four years. RSUs are measured based on the fair market value of the underlying stock on the date of grant and will generally vest over three years. Performance-based stock or cash awards granted under the Amended 2008 Plan are limited to either 500,000 shares of common stock or $1.0 million per recipient per calendar year. The attainment of any performance-based awards granted shall be conclusively determined by a committee designated by the Company’s Board of Directors. At September 30, 2018, 25,000 performance-based stock awards were outstanding. On August 31, 2016, the Company’s Board of Directors adopted the Cerus Corporation Inducement Plan (the “Inducement Plan”), and reserved 1,250,000 shares of its common stock under the Inducement Plan to be used for the issuance of non-statutory stock options and restricted stock units to individuals who were not previously employees or directors of the Company, as an inducement material to the individual’s entry into employment with the Company. The terms and conditions of the Inducement Plan are substantially similar to the Amended 2008 Plan. Effective June 7, 2017, the Company no longer issues shares from the Inducement Plan.

At September 30, 2018, the Company had an aggregate of approximately 23.2 million shares of its common stock subject to outstanding options or unvested RSUs, or remaining available for future issuance under the Amended 2008 Plan and the Inducement Plan, of which approximately 17.6 million shares and 2.0 million shares were subject to outstanding options and unvested RSUs, respectively, and approximately 3.6 million shares were available for future issuance under the Amended 2008 Plan. The Company’s policy is to issue new shares of common stock upon the exercise of options or vesting of RSUs.

Activity under the Company’s equity incentive plans related to stock options is set forth below (in thousands except per share amounts):

	Number of Options Outstanding			Weighted Average Exercise Price per Share
Balances at December 31, 2017		17,138		$	4.27
Granted		3,145			4.61
Exercised		(1,906	)		3.43
Forfeited		(604	)		4.66
Expired		(151	)		5.93
Balances at September 30, 2018		17,622			4.39

Activity under the Company’s equity incentive plans related to RSUs is set forth below (in thousands except per share amounts):

	Number of RSUs Unvested			Weighted Average Grant Date Fair Value per Share
Balances at December 31, 2017		1,256		$	4.53
Granted ⁽¹⁾		1,382			4.45
Vested		(521	)		4.45
Forfeited		(139	)		4.20
Balances at September 30, 2018		1,978			4.52

(1)

Includes the number of shares issuable under the performance-based restricted stock unit awards granted during the nine months ended September 30, 2018.

The Company uses the Black-Scholes option pricing model to determine the grant-date fair value of stock options and employee stock purchase plan rights. The Black-Scholes option pricing model is affected by the Company’s stock price, as well as assumptions regarding a number of complex and subjective variables, which include the expected term of the grants, actual and projected employee stock option exercise behaviors, including forfeitures, the Company’s expected stock price volatility, the risk-free interest rate and expected dividends. The Company recognizes the grant-date fair value of the stock award as stock-based compensation expense on a straight-line basis over the requisite service period, which is the vesting period, and is adjusted for estimated forfeitures.