Stock-Based Compensation

In March 2014, the Company's board of directors approved the 2014 Equity Incentive Plan, or 2014 Plan, under which stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares and units and other cash-based or stock-based awards may be granted to employees, consultants and directors. Shares of common stock that are issued and available for issuance under the 2014 Plan consist of authorized, but unissued or reacquired shares of common stock or any combination thereof.

As of December 31, 2020, a total of 13,322 shares had been reserved for issuance under the 2014 Plan. The 2014 Plan contains a provision that automatically increases the shares available for issuance under the plan on January 1 of each year subsequent to the 2014 Plan's adoption through 2024, by an amount equal to the smaller of (a) 4.5% of the number of shares of common stock issued and outstanding on the immediately preceding December 31, or (b) an amount determined by the Company's board of directors. On January 1, 2021, 2,500 shares were added to the 2014 Plan in accordance with the annual automatic increase provision of the 2014 Plan. In addition, the 2014 Plan reserve is automatically increased to include any shares issuable upon expiration or termination of options granted under the Company's 2007 Stock Plan, or 2007 Plan, for options that expire or terminate without having been exercised. For the nine months ended September 30, 2021, no shares have been transferred to the 2014 Plan from the 2007 Plan, and as of September 30, 2021 a total of 15,823 shares were allocated for issuance under the 2014 Plan. As of September 30, 2021, options to purchase a total of 2,706 shares of common stock have been granted under the 2014 Plan, 6,652 shares have been reserved under the 2014 Plan for the vesting of restricted stock units and market stock units, 1,337 shares have been returned to the 2014 Plan as a result of termination of options that expired or terminated without having been exercised and restricted stock awards that terminated prior to the awards vesting, and 7,802 shares of common stock remain available for future issuance under the 2014 Plan.

In July 2007, the Company adopted the 2007 Plan under which options or stock purchase rights may be granted to employees, consultants and directors. Upon the completion of the Company's initial public offering, or IPO, in March 2014, the board of directors terminated the 2007 Plan in connection with the IPO and all shares that were available for future issuance under the 2007 Plan at such time were transferred to the 2014 Plan. The 2007 Plan will continue to govern the terms and conditions of all outstanding equity awards granted under the 2007 Plan. As of September 30, 2021, no shares remain available for future issuance under the 2007 Plan.

Stock option activity during the nine months ended September 30, 2021 was as follows:


	Number of Options	Weighted Average Exercise Price
Balance as of January 1, 2021	670	$	27.43
Granted	—	—
Exercised	(297)	19.58
Forfeited	—	—
Balance as of September 30, 2021	373	$	33.67

Restricted Stock Units

Restricted stock unit activity during the nine months ended September 30, 2021 was as follows:


	Number of Shares	Weighted Average Grant Date Fair Value
Nonvested as of January 1, 2021	1,757	$	70.74
Granted	662	115.71
Vested	(573)	61.92
Forfeited	(193)	79.28
Nonvested as of September 30, 2021	1,653	$	90.83

Market stock unit activity during the nine months ended September 30, 2021 was as follows:


	Number of Shares	Weighted Average Grant Date Fair Value
Nonvested as of January 1, 2021	370	$	35.67
Granted	101	80.11
Change in awards based on performance(1)	206	39.67
Vested	(386)	33.62
Forfeited	(21)	44.67
Nonvested as of September 30, 2021	270	$	57.58

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(1) Represents the change in the number of MSUs earned based on performance achievement for the performance period.