PP2A is a heterotrimeric type 2 Ser/Thr phosphatase, composed of a structural subunit (A), a regulatory subunit (B), and a catalytic subunit (C). The structural A and catalytic C subunits form the constitutive core of the enzyme, which associates with one of a large number of regulatory B subunits. There are multiple isotypes (encoded by distinct genes) of the B subunit, grouped into three families: B/PR55, B'/PR61, and B''/PR72 . The prototypical B (or PR55) family of subunits is encoded by three genes, Balpha, Bbeta and Bgamma . The B' (or B56) family consists of at least seven splice variants derived from five genes. RNA's for the two members of the B'' family, PR72 and PR130, arise by alternative splicing or use of alternate transcription start sites from the same gene. Primary structures as well as functions of B and B' subunits are conserved from yeast to mammals. PTPA is a member of the B' family that is required for activation of PP2A by an undefined mechanism. PP2A is distinguishable from PP2B/Calcineurin and PP2C by its partial activity in the absence of divalent cations, and its high sensitivity to Okadaic Acid. It is also thought to be the principal downmodulator of Ser/Thr kinase cascades.