ERBB2 and breast cancer: Breast cancers are classified into four subgroups depending on the cell surface receptors such as estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2): luminal B (ER+, PR+, HER2+), luminal A (ER+, PR+, HER2−), HER2 overexpression (ER−, PR−, HER2+), and triple-negative breast cancer (TNBC; ER−, PR−, HER2−) [1].