We employed several mouse models of human breast cancer, where the mouse mammary tumor virus (MMTV) promoter directs transcription of the rat neu proto-oncogene [28] or oncogenes encoding either mT [29] or a stable mutant form of β-catenin in mammary epithelial cells to determine their tumor-resident TIC frequency and to learn whether this parameter differed among the models. This evidence concerns the gene ERBB2 and breast cancer.