ESR1 and breast cancer: It has been found that breast cancer can be classified into three main groups according to whether or not the patient's breast cancer cells express estrogen receptor/progesterone receptor, human epidermal growth factor 2 receptor (HER2) [2]: ER- or PR- positive (also known as luminal type breast cancer), HER2 positive cancer and triple-negative breast cancer (TNBC) (ER-/PR-/HER2-).