ERBB2 and breast cancer: According to the expression status of progesterone receptor (PR), estrogen receptor (ER) and human epidermal growth factor receptor 2 (HER2), breast cancer can be divided into four major subtypes: hormone receptor (HR)+/HER2−, HR+/HER2+, HR−/HER2+ and HR−/HER2−, as previous studies have reported [10, 11].