Note 6 - Common Stock Options (Tables)

12 Months Ended

Dec. 31, 2018

Notes Tables

Share-based Compensation, Stock Options, Activity [Table Text Block]

	Options			Exercise Price			Weighted-Average Exercise Price
Options outstanding at December 31, 2015		4,786,577		$0.53	-	$21.99	$	8.56
Granted		1,059,990		7.71	-	19.10		11.72
Exercised		(204,159	)	0.74	-	19.09		5.56
Forfeitures		(453,750	)	0.52	-	16.91		12.79
Options outstanding at December 31, 2016		5,188,658		$0.52	-	$21.99	$	8.49
Granted		1,251,000		4.15	-	12.62		4.85
Exercised		(406,667	)	0.93	-	14.71		2.28
Forfeitures		(729,367	)	0.94	-	21.99		11.90
Options outstanding at December 31, 2017		5,303,624		$2.50	-	$21.99	$	7.69
Granted		1,153,500		2.50	-	5.93		4.32
Exercised		(160,500	)		2.50			2.50
Forfeitures		(761,185	)	2.50	-	19.09		6.12
Options outstanding at December 31, 2018		5,535,439		$2.50	-	$21.99	$	7.30