In general, the protein phosphatase (PP) holoenzyme is a trimeric complex composed of a regulatory subunit, a variable subunit and a catalytic subunit. Four major families of protein phosphatase catalytic subunits have been identified, designated PP1, PP2A, PP2B (calcineurin) and PP2C. An additional protein phosphatase catalytic subunit, PPX (also known as PP4) is a putative member of a novel PP family. The PP2A family comprises subfamily members PP2Aalpha and PP2Abeta. The PP2A catalytic subunit associates with a variety of regulatory subunits. Regulatory subunits include PP2A-A-alpha and -A-beta, PP2A-B-alpha and -B-beta, PP2A-C-alpha and -C-beta, PP2A-B56-alpha, -B56-beta, -B56-gamma and -B56-delta.