Nodularin is a hepatotoxic monocylic pentapeptide produced by cyanobacterium Nodularia spumigena. It is a potent inhibitor of protein phosphatase types 1 (PP1) and 2A (PP2A), exhibiting IC₅₀ values of 1.8 and 0.026 nM, respectively. It inhibits PP2B only at higher concentrations ( IC₅₀ = 1.8 µM).