Okadaic acid is inhibitor of the serine/theorine protein phosphatases PP1 (IC50 = 15-20 nM) and PP2A (IC50 = 0.1 nM), exhibiting much greater activity towards the latter, in numerous cell types. It is non-phorbol type tumor promoter. Okadaic acid has also been described as a non-TPA type potent tumor promoter. These tumor-promoting properties make okadaic acid a very useful tool for studying cellular processes that are regulated by reversible phosphorylation of proteins