ERBB2 and breast cancer: Clinically, BC is broadly categorized into three subtypes depending on the status of hormone receptors and growth factors: estrogen and progesterone receptor positive (ER + and PR+, 70%), human epidermal growth factor receptor 2 positive (HER2+, 15–20%), and triple-negative BC (TNBC, 10–15%) (Fig. 3) [26, 27].