Although Giordano et al. [104] reported that patients with HER2+ metastatic BC have circulating tumor cells (CTCs) undergoing EMT and enrichment for CSC features, additional studies are needed to determine whether EMT-CTCs [105-107] and CSCs have prognostic or predictive value in HER2+ metastatic BC treated with trastuzumab-based therapy. This evidence concerns the gene ERBB2 and breast cancer.