ESR1 and breast cancer: According to immunohistochemical indexes such as estrogen receptor (ER), progesterone receptor (PR), proliferating cell nuclear antigen (Ki-67), and human epidermal growth factor receptor-2 (HER-2), clinicians determine the molecular subtype of breast cancer, namely luminal A, luminal B, Her-2-positive, and triple-negative breast cancer (TNBC) (15–18), where HER2-positive includes HR-negative and HR-positive (15, 19–23).