A hepatotoxic cyclic heptapeptide toxin produced by cyanobacteria. A selective inhibitor of protein phosphatase 1 (PP1) (IC₅₀ = 1.7 nM) and protein phosphatase 2A (PP2A) (IC₅₀ = 0.04 nM). Less sensitive to PP2B and does not inhibit PP2C up to 4 µM.