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Note 6 - Stock-based Compensation (Tables)
 3 Months Ended
Mar. 31, 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    
1,790,000
     $
2.65
      
 
      
 
  
Exercised    
-
      
 
      
 
      
 
  
Forfeited    
(83,433
 )   $
5.82
      
 
      
 
  
Expired    
(6,702
 )   $
2.38
      
 
      
 
  
Outstanding at March 31, 2018    
4,380,937
     $
5.37
      
8.0
     $
678
  
Exercisable at March 31, 2018    
1,716,700
     $
7.76
      
6.0
     $
232
  
Vested or expected to vest at March 31, 2018 (1)    
4,380,937
     $
5.37
      
8.0
     $
678
  
Common Stock Reserved for Future Issuance [Table Text Block]
    March 31,
2018		   December 31,
2017
         
Conversion of Series A Preferred Stock    
22,112,775
      
22,112,775
  
Stock-based compensation awards    
5,073,822
      
3,572,457
  
Employee Stock Purchase Plan    
192,463
      
192,463
  
Total    
27,379,060
      
25,877,695