Type 2 protein serine/threonine phosphatase (PP2A) is a trimer composed of a catalytic C subunit and two regulatory (A and B) subunits; many isoforms for the A and B subunits have been found. PP2A has a high specific activity against the α subunit of phosphorylase kinase and is insensitive to Inhibitor-2. PP2A is distinguishable from PP2B/Calcineurin and PP2C by its partial activity in the absence of divalent cations, and by its high sensitivity to inhibition by okadaic acid. It is also thought to be the principal down-modulator of serine/threonine kinase cascades. PP2B requires Ca2+/calmodulin for activity, and is an activator of NFAT transcription factors. PP2C requires Mg2+, but is insensitive to okadaic acid, microcystin LR, or the calmodulin inhibitor trifluoperazine.