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Summary of significant accounting policies - Net loss per share (Details) - shares
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share    
Total shares of common stock equivalents 3,771,459 15,998,526
Series A convertible preferred stock    
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share    
Total shares of common stock equivalents   612,446
Series B convertible preferred stock    
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share    
Total shares of common stock equivalents   7,235,969
Series C convertible preferred stock    
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share    
Total shares of common stock equivalents   5,179,877
Options to purchase common stock    
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share    
Total shares of common stock equivalents 3,771,459 2,970,234