ERBB2 and breast cancer: BC is traditionally classified according to the expression of hormonal receptors (HR) and human epidermal growth factor receptor 2 (HER2) in three subtypes: HR-positive (HR+)/HER2-negative (HER2-) BC, HER2+ BC, and HR-/HER2- or triple-negative breast cancer (TNBC), which account for approximately 70%, 15–20%, and 10–15% of all BC diagnoses, respectively [3].