Post-translational phosphorylation is one of the most common protein modifications that occur in animal cells. The vast majority of phosphorylation occurs as a mechanism of acute and reversible regulation of protein function. Studies of mammalian cells metabolically labeled with p32 orthophosphate suggest that as many as one-third of all cellular proteins are covalently modified by protein phosphorylation. Covalent attachment of a phosphate group to an amino acid side chain of a protein can cause a structural change, for example, by attracting a cluster of positively charged side chains. Such a change occurring at one site in a protein can in turn alter the protein’s conformation elsewhere. Reversible protein phosphorylation is the predominant strategy used to control the activity of proteins in eukaryotic cells. The phosphates are transferred from ATP molecules by protein kinases and are taken off by protein phosphatases. In animal cells, serine, threonine and tyrosine are the amino acids subjected to phosphorylation. The protein kinases belong to a large family of enzymes, which contain a similar amino acid catalytic (kinase) domains.