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 familie