The identification of protein phosphorylation as a regulatory mechanism originated from studies by Fischer and Krebs in the mid 1950s that later earned them the 1992 Nobel prize. It is the major mechanism for the regulation of diverse cellular processes including cell division, protein synthesis, transcriptional regulation and neurotransmission. The steady state phosphorylation of any given substrate is governed by the opposing activities of kinases and phosphatases. It is now believed that a third of all eukaryotic cellular proteins are phosphorylated and that the majority of all phosphorylation events occur on serine and threonine residues (greater than 95%).