The Company’s 2017 Stock Incentive Plan (the “2017 Plan”) provides for the Company to sell or issue common stock or restricted common stock, or to grant qualified incentive stock options, nonqualified stock options, SARs, RSUs or other stock-based awards to the Company’s employees, officers, directors, advisers and outside consultants. The total number of shares authorized for issuance under the 2017 Plan was 13,500 shares as of June 30, 2019, of which 10,739 shares remained available for future grant.

The following table summarizes the outstanding stock option activity and a summary of information related to stock options as of and for the six months ended June 30, 2019:

	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding at January 1, 2019		9,566		$	7.29		6.35	$	61,561
Granted		94			7.23
Exercised		(872	)		2.49
Forfeited		(139	)		13.29
Outstanding at June 30, 2019		8,649		$	7.68		6.06	$	13,679
Options exercisable at June 30, 2019		6,839		$	6.08		5.52	$	13,638
Vested or expected to vest at June 30, 2019		8,553		$	7.61		6.04	$	13,675

The fair value of each option is estimated on the date of grant using the Black-Scholes option-pricing model using the following assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
Risk-free interest rate	2.0%	2.9%	2.0%–2.5%	2.7%–2.9%
Expected term (in years)	6.1	6.1–6.2	6.1–6.2	6.0–6.2
Expected volatility	29.1%	32.0%–32.1%	29.1%–30.6%	32.0%–32.6%
Expected dividend yield	0.0%	0.0%	0.0%	0.0%

The weighted-average grant-date fair value of options granted during the six months ended June 30, 2019 and 2018 was $2.45 and $8.76 per share, respectively. Cash proceeds received upon the exercise of options were $2,168 and $10,147 during the six months ended June 30, 2019 and 2018, respectively. The intrinsic value of stock options exercised during the six months ended June 30, 2019 and 2018 was $6,087 and $83,861, respectively. The aggregate intrinsic value is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock.

A summary of RSU activity under the Company’s 2011 Stock Incentive Plan (the “2011 Plan”) and the 2017 Plan for the six months ended June 30, 2019 is as follows:

	Number of Shares			Weighted- Average Grant Date Fair Value		Aggregate Fair Value
Unvested balance at January 1, 2019		759		$	13.40
Granted		1,117			10.90
Vested		(271	)		9.90	$	3,305
Forfeited		(56	)		14.15
Unvested balance at June 30, 2019		1,549		$	12.16

The Company withheld 76 shares of common stock in settlement of employee tax withholding obligations due upon the vesting of RSUs during the six months ended June 30, 2019. During the six months ended June 30, 2018, the Company withheld no shares of common stock in settlement of employee tax withholding obligations due upon the vesting of RSUs.

In January 2017, the Company granted 110 SARs that allow the holder the right, upon exercise, to receive in cash the amount of the difference between the fair value of the Company’s common stock at the date of exercise and the price of the underlying common stock at the date of grant of each SAR. The price of the underlying common stock on the date of grant was $12.24 per share and the grant-date fair value was $4.52 per SAR. The SARs vest over a four-year period from the date of grant and expire ten years from the date of grant. As of June 30, 2019, 240 SARs were outstanding and 19 were unvested. As of June 30, 2019, there were 221 SARs exercisable and the weighted-average fair value was $2.42 per SAR. The fair value of the SAR liability as of June 30, 2019 and December 31, 2018 was $521 and $1,387, respectively, (see Note 6) and was included in accrued expenses and other current liabilities in the accompanying condensed consolidated balance sheets.

Stock-based compensation expense related to stock options, RSUs and SARs for the three and six months ended June 30, 2019 and 2018 was classified in the condensed consolidated statements of operations and comprehensive income (loss) as follows:

	Three Months Ended June 30,					Six Months Ended June 30,
	2019		2018			2019		2018
Cost of revenue	$	53	$	(72	)	$	124	$	137
Research and development expenses		502		(1,016	)		643		1,002
Selling, general and administrative expenses		2,104		1,182			3,792		3,185
	$	2,659	$	94		$	4,559	$	4,324

As of June 30, 2019, there was $23,099 of unrecognized compensation cost related to outstanding stock options, RSUs and SARs, which is expected to be recognized over a weighted-average period of 2.79 years.