ESR1 and breast cancer: 1) Female patients diagnosed with breast cancer, with positive estrogen receptor (ER) and/or progesterone receptor (PR) by immunohistochemical examination of postoperative pathology; 2) age between 18 and 65 years; 3) patients who have undergone surgery, chemotherapy, radiotherapy, and/or targeted therapy; 4) patients who have received aromatase inhibitor treatment for at least 2 months; 5) patients who voluntarily underwent clinical investigation and related examinations and provided signed informed consent.