ERBB2 and breast cancer: This predictive gene signature was obtained while taking two important elements into account: (a) HER2 overexpression drives mammary carcinogenesis, via its effects on normal and malignant mammary stem cells; (b) in a mouse mammary model of HER2+ breast cancer, mammary stem cells (CD24+/JAG1-) have been purified and used to generate a 17-gene signature [86].