ERBB2 and breast cancer: Specifically, because CD44+CD24−/low mesenchymal-like CSCs distinctively possess a highly endocytic activity, the otherwise irrelevant HER2 can open the door to a “Trojan horse” type approach through the employment antibody-drug conjugates such as T-DM1, which will allow a rapid and CSC-targeted delivery of cytotoxic drugs to trastuzumab-unresponsive basal/cHER2+ BC (Figure 4).