ERBB2 and cancer: Many mechanisms can contribute to the transition of HR+/luminal A breast tumor cells towards a more advanced phenotype, with roles attributed to progenitor and cancer stem cells (CSCs) and to increased stemness in this process; in this context, recent studies have shown that mammary CSCs of luminal origin can differentiate in vivo to the highly aggressive subtypes TNBC, claudin-low and HER2+ [9,10,11].