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Note 8 - Net Loss Per Share Attributable to Common Stockholders (Tables)
3 Months Ended
Mar. 31, 2018
Notes Tables  
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
    Three Months Ended March 31,
    2018   2017
Outstanding stock options    
4,380,937
     
2,751,595
 
Outstanding ESPP shares    
55,716
     
91,740
 
Convertible preferred stock    
22,112,775
     
-
 
     
26,549,428
     
2,843,335