PCR-based genotype analysis using DNA samples prepared from the mammary gland tumors containing both cancer and non-cancer cells detected both the knockout Pten and Tp53 alleles in cancer cells infected by RCAS-Cre virus, and the floxed Pten and Tp53 alleles in non-cancer cells that were not infected by RCAS-Cre virus (Fig. 2A-B). The gene discussed is TP53; the disease is cancer.