ERBB2 and breast cancer: Breast cancer is categorized into five molecular subtypes according to the presence or absence of estrogen or progesterone receptors and human epidermal growth factor receptor 2 (HER2) and Ki67 expression levels: luminal A, luminal B, luminal B plus HER2 overexpression, HER2‐enriched, and triple‐negative subtypes [5, 6, 7].