ERBB2 and breast cancer: In summary, targeting of Erbb2/Neu activation to specific cell populations within the mouse mammary epithelium supports the role of luminal ER− stem/progenitor cells as the cells of origin for most breast cancer subtypes: TNBC/‘basal-like’, luminal ER+ (Melchor et al., 2014; Molyneux et al., 2010) and now HER2+.