Any of a family of proteins that can add a phosphate group to specific tyrosine residues of other proteins; they are homologous to the Src protein (pronounced ‘Sark’) encoded by the viral oncogene v-src, which can produce tumours in chickens. Commonly associated with cell receptors, these kinases are involved in signal transduction pathways controlling cell division and proliferation and antigen recognition by lymphocytes. They contain the prototypical SH2 domain (Src homology 2 domain) and the SH3 domain, which binds to proline-rich regions in other proteins.