protein phosphorylation analysis is the most commonly studied area of post-translational modification since it plays a vital role in intracellular signal transduction and is involved in regulating cell cycle progression, differentiation, transformation, development, peptide hormone response, and adaptation. It has been estimated that one third of mammalian proteins may be phosphorylated and this modification often plays a key role in modulating protein function. Reversible protein phosphorylation, principally on serine, threonine or tyrosine residues, is one of the most important and well-studied post-translational modifications.