This study is a prospective non-interventional real-world study enrolling patients with advanced breast cancer who are ER+/HER2- and have ESR1- gene mutations, collecting information on patients' complaints, physical examination, laboratory tests, imaging tests and adverse events to observe the safety of elacestrant treatment.

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

Advanced breast cancer with ER+/HER2- and ESR1- gene mutation