This is a prospective, non-interventional real-world study to observe the efficacy and safety of different treatment regimens in patients with ESR1-mutated HR+/HER2-advanced breast cancer after failure of endocrine therapy.

Epidemiological data, efficacy and safety measures will be collected for each subject. Data on efficacy and safety assessment indicators will be collected every 2-3 months until disease progression, receipt of a new anti-tumour treatment modality, death, loss to follow-up, and arrival at the data collection cut-off date. The cut-off date for data collection is defined as 8 weeks after completion of 6 visits for each subject, or 4 weeks after treatment discontinuation and subject discontinuation/withdrawal. Subjects receiving a different treatment regimen remained subject to assessment of safety indicators 4 weeks after discontinuation of the original treatment regimen.

Inclusion Criteria:

• 1. must have a histologically or cytologically confirmed diagnosis of breast cancer with evidence of locally advanced disease unsuitable for excision or radical radiotherapy, or evidence of metastatic disease unsuitable for radical treatment.
• 2. female ≥ 18 years of age
• 3. female subjects must be postmenopausal (meeting any of the following criteria is sufficient) a) Has undergone oophorectomy. b) Age ≥ 60 years. c) 40 years old < age ≤ 60 years old with 1 year of menopause. d) Age <60 years and receiving ovarian suppression therapy.
• 4. ER-positive and HER2-negative status and ESR1-mutation positive must be confirmed.
• 5. must have progressed on at least one line of endocrine therapy prior to enrollment, including monotherapy or combination therapy.
• 6. have normal organ function (as assessed by the investigator).

Exclusion Criteria:

• 1. women who are pregnant or breastfeeding
• 2. known difficulties in tolerating oral medications, or conditions that interfere with the absorption of oral medications or allergies to medications and their excitements
• 3. other conditions that make enrollment in the study unsuitable, at the discretion of the investigator

Patients with HR+/HER2-advanced breast cancer with ESR1 mutation after failure of endocrine therapy