An oligopeptide, often just called peptide (, "a few"), consists of two to twenty amino acids and can include dipeptides, tripeptides, tetrapeptides, and pentapeptides. There have been more than 600 oligopeptide variants described, and about half of them are separated into seven classes (based on molecular structure): aeruginosins, cyanopeptolins, microcystins, microviridins, microginins, anabaenopeptins and cyclamides. Microcystins are best studied, because of their potential toxicity impact in drinking water. A review of some oligopeptides found that the largest class are the cyanopeptolins (40.1%), followed by microcystins (13.4%).