ERBB2 and breast cancer: Based on the expression of hormone receptors (HR), including the estrogen receptor (ER), progesterone receptor (PR), and the human epidermal growth factor receptor 2 (HER-2), breast cancer can be classified into four subtypes including HR + HER2 −, HR + HER2 + , HR-HER2 + , and HR-HER2 − [6].