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Note 6 - Stock-based Compensation (Tables)
 6 Months Ended
Jun. 30, 2018
Notes Tables  
Share-based Compensation, Stock Options, Activity [Table Text Block]
    Options   Weighted-Average Exercise Price   Weighted-Average Remaining Contractual Term (years)   Aggregate Intrinsic Value
                 
Outstanding at December 31, 2017    
2,681,072
     $
7.18
      
6.8
     $
121
  
Granted    
1,891,600
     $
2.65
      
 
      
 
  
Exercised    
-
     $
-
      
 
      
 
  
Forfeited    
(83,433
 )   $
5.82
      
 
      
 
  
Expired    
(42,013
 )   $
13.30
      
 
      
 
  
Outstanding at June 30, 2018    
4,447,226
     $
5.22
      
7.9
     $
614
  
Exercisable at June 30, 2018    
1,844,871
     $
7.44
      
6.1
     $
286
  
Vested or expected to vest at June 30, 2018 (1)    
4,447,226
     $
5.22
      
7.9
     $
614
  
Common Stock Reserved for Future Issuance [Table Text Block]
    June 30,   December 31,
    2018   2017
         
Conversion of Series A Preferred Stock    
22,112,775
      
22,112,775
  
Stock-based compensation awards    
5,163,957
      
3,572,457
  
Employee Stock Purchase Plan    
140,479
      
192,463
  
Total    
27,417,211
      
25,877,695