Common Stock and Stockholders’ Equity (Deficit) (Tables)

6 Months Ended

Jun. 30, 2016

Common Stock and Stockholders’ Equity (Deficit)

Summary of activity under the Plan

Activity under the Plan is as follows (in thousands, except share and per share amounts and years):

					Weighted-
			Weighted-		Average
	Shares	Number of	Average		Remaining	Aggregate
	Available	Shares	Exercise		Contractual	Intrinsic
	for Grant	Outstanding	Price		Life in Years	Value
Balance at December 31, 2015	1,986,390	3,851,095	$	7.62	8.54	$	41,894
Increase in Plan authorized shares	1,940,154	—	$	—	—	$	—
Stock option grants	(2,151,541)	2,151,541	$	—	—	$	—
Stock options exercised	—	(219,266)	$	—	—	$	—
Stock options expired	94,352	(94,352)	$	—	—	$	—
Balance at June 30, 2016	1,869,355	5,689,018	$	9.64	8.72	$	39,485
Vested or expected to vest June 30, 2016		5,331,251	$	9.52	8.68	$	37,630
Exercisable as of June 30, 2016		1,463,897	$	4.30	7.45	$	17,153

Assumptions used for estimate of fair value of options

	Six Months Ended June 30,
	2016		2015
Volatility		44.7% – 46.0%		49.6% – 51.0%
Expected life (in years)		6.0		7.0
Risk-free interest rate		1.24% - 1.91%		1.58% - 2.06%
Dividend yield		–		–
Weighted-average fair value of underlying common stock	$	5.71	$	5.15

Components of operating expense charged for compensation cost expense

Total compensation costs charged as an expense for stock‑based awards, including stock options, recognized in the components of operating expenses are as follows (in thousands):

	Quarters Ended				Six Months Ended
	June 30,				June 30,
	2016		2015		2016		2015
Administrative and marketing	$	115	$	23	$	217	$	41
Sales		228		95		438		165
Technology and development		266		52		475		92
General and administrative		1,025		397		1,792		1,080
Total stock-based compensation expense	$	1,634	$	567	$	2,922	$	1,377