A summary of the Company’s stock option activity and related information is as follows:

				Options Outstanding
	Shares Available for Grant			Number of Stock Options Outstanding			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
											(In thousands)
Balance at December 31, 2018		1,908,626			11,603,708		$	6.64		7.6	$	10,151
Additional options authorized		3,853,280
Options granted		(2,624,505	)		2,624,505			8.19
Options exercised		—			(2,219,251	)		4.34
Options canceled		553,585			(553,585	)		6.73
Withheld shares to pay for taxes on RSU released shares		253,986
RSUs granted		(708,144	)
RSUs canceled		70,732
Balance at September 30, 2019		3,307,560			11,455,377		$	7.58		7.9	$	1,011
Vested and exercisable at September 30, 2019					4,575,393		$	6.25		6.2	$	1,010
Vested and expected to vest at September 30, 2019					10,793,250		$	7.38		7.7	$	1,010

The weighted-average grant date fair value of options granted to employees were $4.71 and $5.02 per share during the nine months ended September 30, 2019 and 2018, respectively. The grant date fair values of options vested were $11.4 million and $4.3 million during the nine months ended September 30, 2019 and 2018, respectively.

Aggregate intrinsic value represents the difference between the estimated fair value of the underlying common stock and the exercise price of outstanding, in-the-money options. The aggregate intrinsic values of options exercised were $7.8 million and $11.6 million during the nine months ended September 30, 2019 and 2018, respectively.

At September 30, 2019, total unrecognized compensation cost related to stock options granted to employees, net of estimated forfeitures, was $29.0 million which is expected to be recognized over a weighted-average period of 2.9 years.

The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the estimated fair value of the Company’s common stock, as well as assumptions regarding a number of complex and subjective variables. The variables used to calculate the fair value of stock options using the Black-Scholes option-pricing model include actual and projected employee stock option exercise behaviors, expected price volatility of the Company’s common stock, the risk-free interest rate and expected dividends. Each of these inputs is subjective and generally requires significant judgment to determine.

The fair value of employee stock options is estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:

From time to time, the Company grants Restricted Stock Units, or RSUs, to its Board of Directors and certain employees for their services. The RSUs granted to board members are either fully vested upon issuance or vest over a period of time from the grant date and will be released and settled upon termination of the board member’s services or the occurrence of a change in control event. In January 2019, the Company granted RSUs to its Board of Directors as part of the director compensation program. In March 2019, the Company began granting RSUs to certain employees. These RSUs vest in equal annual installments over either two or three years from the grant date and are subject to the participants continuing service to the Company over that period. The fair value of RSUs is based on the closing market price of the Company’s common stock on the grant date.

	RSUs
	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2018		1,736,234		$	9.65
RSUs granted		708,144			7.79
RSUs vested		(631,252	)		9.40
Unvested at September 30, 2019		1,813,126		$	9.13
Vested and unreleased		104,173
Outstanding at September 30, 2019		1,917,299

The total grant date fair value of RSUs awarded was $5.5 million for the nine months ended September 30, 2019. The total fair value of RSUs vested was $5.9 million for the nine months ended September 30, 2019. During the nine months ended September 30, 2019, the RSUs that vested were net share settled such that the Company withheld shares with value equivalent to the employees’ obligation for the applicable income and other employment taxes and remitted the cash to the appropriate taxing authorities. The total shares withheld were approximately 0.3 million and were based on the value of the RSUs on their respective vesting dates as determined by the Company’s closing stock price. Total payments for the employees’ tax obligations to taxing authorities were $2.4 million for the nine months ended September 30, 2019 and are reflected as a financing activity within the Condensed Consolidated Statement of Cash Flows. These net share settlements reduced the number of shares that would have otherwise been issued as a result of the vesting and did not represent an expense to the Company. For the nine months ended September 30, 2018, the Company granted 1.8 million shares of RSUs and recorded $1.2 million in stock-based compensation expense related to the issuance of RSUs.

As of September 30, 2019, total unrecognized stock-based compensation cost related to RSUs was $12.7 million, which is expected to be recognized over a weighted-average period of 1.9 years. As of September 30, 2019, 1,617,702 RSUs are expected to vest.

Total stock-based compensation expense recognized in the Company’s condensed consolidated statements of operations and comprehensive loss is classified as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	377	$	511	$	1,221	$	1,061
Selling and marketing		368		234		980		555
General and administrative		4,165		6,434		12,168		8,436
Total stock-based compensation expense	$	4,910	$	7,179	$	14,369	$	10,052

During the three and nine months ended September 30, 2019 and 2018, there was no stock-based compensation expense capitalized as a component of inventory or recognized in cost of revenue. Stock-based compensation relating to stock-based awards granted to consultants was insignificant during the three and nine months ended September 30, 2019 and 2018.