<p>Protein phosphorylation plays a central role in the regulation of cell functions [<cite idref="PUB00000291"/>], causing the activation or inhibition of many enzymes involved in various biochemical pathways [<cite idref="PUB00005960"/>]. Kinases and phosphatases are the enzymes responsible for this, and may themselves be subject to control through the action of hormones and growth factors [<cite idref="PUB00000291"/>]. Serine/threonine(S/T) phosphatases (<db_xref db="EC" dbkey="3.1.3.16"/>) catalyse the dephosphorylation of phosphoserine and phosphothreonine residues. In mammalian tissues four different types of PP have been identified and are known as PP1, PP2A, PP2B and PP2C. Except for PP2C, these enzymes are evolutionary related. The catalytic regions of the proteins arewell conserved and have a slow mutation rate, suggesting that major changes in these regions are highly detrimental [<cite idref="PUB00000291"/>].</p> <p>Protein phosphatase-1 (PP1) and protein phosphatase-2A (PP2A) have a broad specificity and there are two closely related isoforms of each, alpha and beta. PP2A is a trimeric enzyme that consists of a core composed of a catalytic subunit associated with a 65 kDa regulatory subunit and a third variable subunit. Protein phosphatase-2B (PP2B or calcineurin), a calcium-dependent enzyme whose activity is stimulated by calmodulin, is composed of two subunits the catalytic A-subunit and the calcium-binding B-subunit. The specificity of PP2B is restricted. Other serine/threonine specific protein phosphatases that have been characterised include mammalian phosphatase-X (PP-X), and Drosophila phosphatase-V (PP-V), which are closely related but yet distinct from PP2A; yeast phosphatase PPH3, which is similar to PP2A, but with different enzymatic properties; and Drosophila phosphatase-Y (PP-Y), and yeastphosphatases Z1 and Z2 which are closely related but yet distinct from PP1.</p>