We have reported the use of the RCAS-TVA system to model sporadic tumorigenesis upon oncogenic activation in somatic mammary epithelial cells in the mouse. Here we review the advantages of this approach as compared to conventional mouse models with transgenic oncogene expression. We also in detail describe the RCAS-TVA method for introducing genes into somatic mammary epithelial cells engineered to express the avian receptor tva. This method may be particularly useful in modeling oncogenic activation and subsequent tumorigenesis in distinct breast epithelial cell sub-populations, including progenitor cells.