PP2A core enzyme consists of a 36-kDa catalytic C subunit and a constant 65-kDa regulatory/structural A subunit that interact with either a B regulatory subunit or with cell signaling molecules, that likely modulate substrate selectivity, catalytic activity, and subcellular localization, yielding the trimeric holoenzyme. Combinations of different subunit isoforms can generate many forms of PP2A, which may differ in substrate specificity, subcellular localization, or tissue specific expression. [ ]