Abstract: Reversible protein phosphorylation plays a pivotal role in cellular signal transduction, moderating such diverse functions as neurotransmission, muscle contraction, glycogen synthesis, T-cell activation and cell proliferation.1-5 Serin/threonine phosphatases, which are responsible for protein dephosphorylation. Of the serine/threonine phosphatases, protein phosphatases 1 and 2A (PP1 and PP2A, respectively) share sequence identity between both enzyme subunits (50% for residues 23-292; 43% overall), are present in all eukarytoic cells and are together responsible for 90% of all cellular dephosphorylation. An interesting link between PP1 and PP2A is their shared sensitivity towards a structurally diverse family of natural products: the okadaic acid (1) class of compounds.