ERBB2 and breast cancer: In fact, according to the expression of a few protein biomarkers, BC can be divided into four main subtypes with distinct behavior in terms of prognosis and response to therapy: Luminal A (LA) and Luminal B (LB), both estrogen (ER) and progesterone (PgR) receptors positive, but characterized by a low and high proliferation index respectively, HER2 subtype defined by overexpression/amplification of the HER2 gene and Triple Negative/basal-like (TN) lacking ER, PgR and HER2 expression.