ERBB2 and breast cancer: According to the expression of human epidermal growth factor receptor 2 (HER2) and hormone receptors (ER and PR), breast cancer is classified into four major molecular subtypes, namely, luminal A (HR+/HER2-), luminal B (HR+/HER2+), HER2+, and triple-negative breast cancer (ER-, PR-, and HER2-negative) (2).