ERBB2 and breast cancer: By using the PAM50 test, Agostinetto et al. and Schettini et al. demonstrated that HER2-Low breast cancers are composed of heterogeneous clusters of tumor cells, including 50.8–56.9% Luminal-A, 22.8–28.8% Luminal-B, 13.3–17.7% basal-like and even 3.5–3.6% HER2-enriched tumors [29,30].