The human RON and its mouse homologue stk are members of the MET family of tyrosine kinase receptors. We have previously shown that the RON gene is over-expressed in human breast carcinomas. As cat mammary tumours have been proposed as a suitable model for aggressive human breast cancer, we identified the feline stk gene and studied its expression in cat mammary cancer. Feline stk sequences were found highly homologous to the stk and RON gene exons that encode the juxtamembrane and transmembrane domains of the stk and RON receptors. Feline stk-specific transcript was detected by RT-PCR in cat lung and in 7/8 feline mammary carcinomas and a synchronous skin metastasis examined. Western blot and immunohistochemical analyses were carried out with an antibody that recognized both the human RON and mouse stk receptors. This antibody specifically detected a 135 Kd feline protein and stained 10/34 mammary carcinoma archival samples. These data show that the pattern of expression and distribution of the stk protein in feline mammary cancer could be superimposed on that of the RON receptor in human breast cancer and suggest that these feline tumours are a suitable model to test innovative approaches to therapy of aggressive human breast carcinomas.