ESR1 and breast cancer: Gallen International Conference on Breast Cancer in 2015 (44), based on the estrogen receptor (ER) progesterone receptor (PR), human epidermal growth factor receptor-2 (human epidermalgrowth factor), the expression of receptor-2, HER2) and Ki-67 proliferation index in tumor tissues is different, and breast cancer is divided into 4 molecular subtypes.