Src (also known as pp60src) is a non receptor Tyrosine Kinase involved in signal transduction in many biological systems and implicated in the development of human tumors. There are two critical phosphorylation sites of tyrosine on Src, tyrosine 418 and tyrosine 529 (referring to human Src sequence). The tyrosine 418 is located in the catalytic domain and is one of the autophosphorylation sites. Full catalytic activity of Src requires phosphorylation of tyrosine 418. This region of Src is also highly conserved in all of the related Src-family kinases, and thus prior immunoprecipitation may be required to identify which Src family member is being activated.