ERBB2 and breast cancer: According to an immunohistochemistry (IHC) classification system, BC can be divided into four subtypes, basal-like, HER2-enriched, and luminal A and B subtypes, on the basis of the expression of progesterone receptor (PR), estrogen receptor (ER), human epidermal growth factor receptor 2 (HER2), and Ki-67 (2).