ESR1 and breast cancer: Studies have shown that status of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), circulating tumor cell DNA (ctDNA), carcinoembryonic antigen (CEA), carbohydrate antigen 15−3 (CA15-3), extracellular vesicles (EV), circulating miRNA, BRCA gene mutations and other biomarkers were closely related to diagnosis and prognosis of breast cancer, but without specificity and stability [4–10].