A diverse array of proteins involved in every aspect of cellular activity including signaling, division, homeostasis, and differentiation is modified by phosphorylation. Altering the phosphorylation state of protein through the activation of specific kinases and phosphatases may result in a change in the protein’s function, localization, or metabolism. The amino acids that are phosphorylated in eukaryotlc cells are serine, threonine, and tyrosine, although in very rare circumstances histidine (Rose et al., 1975; Huebner and Matthews, 1985) and arginine (Levy-Favatier et al., 1987; Wakim and Aswad, 1994) may also be targets. Phosphorylation is catalyzed by protein kinases that can be divided into three general categories: serine/threonine protein kinases, tyrosine protein kinases, and dual-specificity protein kinases. The superfamily of serine/threonine protein kinases can be further divided into proline-directed and nonproline-directed protein kinase families (Hardie and Hanks, 1995a). Unlike the other protein kinases, dual specificity protein kinases are capable of phosphorylating tyrosine, serine, and threonine residues of a protein (for review, see Hardie and Hanks, 1995a,b). Protein phosphatases that catalyze the dephosphorylation of proteins are categorized into two main groups: serine/ threonine protein phosphatases and tyrosine protein phosphatases.