Src protein, coloured transmission electron micrograph (TEM). Src is a tyrosine kinase, a signalling protein in cells that has the ability to insturn onins protein synthesis and cellular growth. Overexpression of the gene that codes for the protein has been linked with breast, ovarian and colon cancer. The link with cancer was first discovered by its connection with the Rous sarcoma virus that causes tumours in chickens. It injects a viral form of the protein, called v-Src, into cells that is always active. Mutations in the cellular form of Src (c-Src) are thought to induce the same effect and researchers are looking for ways to stop these mutated kinases and restore the normal limits on growth.