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. Microcystin-LR is a selective inhibitor of protein phosphatase 2A (PP2A) (IC50= 0.04 nM) and will completely inhibit this enzyme without affecting PP1 when used at a concentration of 0.5 nM. The PP1 IC50 is about 1.7 nM. Microcystins are at least 10 times more potent as serine/threonine PP inhibitors than okadaic acid, another microalgal toxin also used for this purpose.