Purity >95%. Microcystins are hepatotoxic cyclic heptapeptide toxins produced by cyanobacteria. They are responsible for periodic poisonings of humans and livestock drinking fresh water where the blue-green algae are endemic. The toxicity of microcystins is initiated by the inhibition of the catalytic activity of protein phosphatase (PP) 1 and PP2A. Microcystin-RR inhibits PP1C, PP1Gamma, and PP2A with Ki values of 0.15, 0.24, and 0.018 nM, respectively.