Protein tyrosine kinases (PTKs) are enzymes that catalyze the transfer of the-phosphate group of ATP to tyrosine residues of protein substrates. PTKs can be subdivided into two large families, receptor tyrosine kinases (RTKs, e.g. EGF, FGF, PDGF, VEGF) and nonreceptor tyrosine kinases (e.g. Src, JAKs, Abl).