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		(1,872,022	)		1,872,022			8.28
Options exercised		-			(588,120	)		4.90
Options canceled		79,293			(79,293	)		7.49
RSUs granted		(651,392	)
Balance at March 31, 2019		3,317,785			12,808,317		$	6.95		7.9	$	16,290
Vested and exercisable at March 31, 2019					4,071,182		$	4.32		5.9	$	12,759
Vested and expected to vest at March 31, 2019					12,181,953		$	6.89		7.9	$	16,094

The weighted-average grant date fair value of options granted to employees was $4.78 and $4.75 per share during the three months ended March 31, 2019 and 2018, respectively. The grant date fair value of options vested was $1.2 million and $1.1 million during the three months ended March 31, 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 value of options exercised was $1.9 million and $1.6 million during the three months ended March 31, 2019 and 2018, respectively.

At March 31, 2019, total unrecognized compensation cost related to stock options granted to employees, net of estimated forfeitures, was $34.1 million which is expected to be recognized over a weighted-average period of 3.2 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 option 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 for their services. These RSUs 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 granted RSUs to several employees. 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		651,392			7.69
RSUs vested		(21,149	)		6.33
Unvested at March 31, 2019		2,366,477		$	9.14
Vested and unreleased		142,656
Outstanding at March 31, 2019		2,509,133

The total grant date fair value of RSUs awarded was $5.0 million for the three months ended March 31, 2019. The total fair value of RSUs vested was $0.1 million for the three months ended March 31, 2019. There were no RSUs awarded or vested in the three months ended March 31, 2018.

As of March 31, 2019, total unrecognized stock-based compensation cost related to RSUs was $16.2 million, which is expected to be recognized over a weighted-average period of 2.3 years. As of March 31, 2019, 2,191,333 shares of 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 March 31, 2019
	2019		2018
Research and development	$	391	$	203
Selling and marketing		265		127
General and administrative		3,896		860
Total stock-based compensation expense	$	4,552	$	1,190

During the three months ended March 31, 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 months ended March 31, 2019 and 2018.