ESR1 and breast cancer: According to the expression of estrogen receptor (ER), progesterone receptor and human epidermal growth factor receptor 2 (HER‐2), breast cancer is identified as four molecular subtypes: basal‐like, HER‐2 positive, luminal A and luminal B, with the first two subtypes from negative ER tumors and the last two from positive ER tumors [41].