Members of the Src family of protein tyrosine kinases are involved in a variety of cellular processes, including cell growth, cell differentiation and neuronal signalling. N-terminal to the catalytic domain, Src family members contain a Src homology 2 (SH2) domain, a Src homology 3 (SH3) domain, and a unique domain, all capable of protein-protein interactions. Negative regulation by phosphorylation of a conserved tyrosine residue at the C-terminal tail of the molecules is characteristic of this family of enzymes. Phosphorylation of this residue causes the intramolecular interactions of the SH2 domain with the tail, and of the SH3 domain with an as yet undefined region, probably within the catalytic domain. Enzymatically active Src family kinases, on the other hand, are phosphorylated at a tyrosine in the middle of the catalytic domain and phosphorylation of this residue is a prerequisite for high activity. Regulators of these enzymes may thus act by altering the phosphorylation state of the two key tyrosine residues or by interfering with the regulatory intramolecular interactions, either by direct binding or by modification of the interfaces involved.