The phosphorylation and de phosphorylation of proteins on serine and threonine residues is an essential means of regulating a broad range of cellular functions in eukaryotes, including division, homeostasis and apoptosis. A geoup of proteins that are intimately involved in this process are the serine/ threonine protein phosphatases. The protein phosphatases (PP) holoenzyme is a trimertic complex composed of a regulatory subunit, a variable subunit and a catalytic subunit. Four major families of protein phosphatase catalytic subunits have been identified and are designated PP1, PP2A, PP2B (calcineurin) and PP2C. An additional protein phosphatase catalytic subunit, PPX or PP4, is the member of a potentially novel PP family. The PP1 family is composed of subfamily members PP1 alpha, PP1 beta,and PP1 gamma.The PP2A family iscomposed of subfamily members PP2A alpha,and PP2A beta. The PP2B family is composed of subfamily members PP2B-A alpha, PP2B-A beta and PP2B-A gamma. The PP2C family is composed of PP2C alpha, PP2C beta, PP2C gamma. Wip1, a protein isentified in the p53 DNA response pathway, has also been identified as a potential member of the PP2C family. Regulatary subunits include dnuclear inhibitor of PP1(NIPP1), PP2A-AAlphaandA beta, PP2A-B alpha and B beta,PP2A-C alpha and C beta, PP2A-B56-alpha andB56 beta, PR48 and PP2B-B1 andB2.