Stockholders’ Equity

Common Stock Offering

April 2018,

we sold

6.9

million shares of our common stock at a public offering price of

$26.00

per share. We received total net cash proceeds of

$168.7

million (net of

$10.7

million in issuance costs) from the sale of the common stock.

June 2018,

we sold

5.1

million shares of our common stock at a public offering price of

$39.50

per share. We received total net cash proceeds of

$194.4

million (net of

$5.6

million in issuance costs) from the sale of the common stock.

Stock Option

Activity

The following table summarizes the stock option activity for the

three

months ended

March 31, 2019:

						Weighted-
						a verage
				Weighted-		Remaining
				a verage		Contractual
	Shares			Exercise		Term
	(in thousands)			Price		(Years)
Balance at December 31, 2018		15,265		$	18.33		6.87
Granted		253		$	25.99
Exercised		(732	)	$	8.93
Expired and forfeited		(505	)	$	21.11
Balance at March 31, 2019		14,281		$	18.85		7.19

For the

three

months ended

March 31, 2019,

732,000

shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of

$6.5

million.

Stock-based Compensation

The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):

	Three Months Ended March 31,
	2019		2018

Research and development	$	5,324	$	3,036
General and administrative		3,680		2,276
Sales and marketing		8,898		2,389
Total stock-based compensation expense	$	17,902	$	7,701

As of

March 31, 2019,

there was

$100.3

million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of

2.6

years.

We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:

	March 31,
	20 19			20 18
Risk-free interest rate		2.5	%		2.6	%
Dividend yield		0.0	%		0.0	%
Volatility		68.8	%		71.4	%
Expected life (years)		6			6

We estimated the fair value of each purchase right granted under our

1997

Employee Stock Purchase Plan at the beginning of each new offering period using the Black-Scholes option pricing model. There were

new offering periods for the

three

months ended

March 31, 2019

and

2018.