ERBB2 and breast cancer: Breast cancer is categorized into four primary clinically relevant molecular subtypes based on the prognosis and levels of gene expression: luminal A or hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2−), luminal B or HR+ /HER2+, HER2+, and triple-negative breast cancer (TNBC) or HR−/HER2− [4–7].