ESR1 and breast cancer: Nevertheless, based on the expression of immunohistochemistry (IHC) markers, such as estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), BC cancer can be split into four major subtypes; namely, ER+/PR+/HER2−, ER+/PR+/HER2+, ER-/PR-/HER2+ and ER-/PR-/HER2−.