ESR1Real World Assessment Prevalence Study

Inclusion Criteria: 1. Subject must be female or male and aged ≥ 18 years on the day of signing informed consent 2. Patient has locally advanced or metastatic breast cancer not amenable to curative treatment 3. Patient has HER2- breast cancer confirmed by local laboratory, defined as a negative in situ hybridization test or an IHC status of 0 or 1+. If IHC is 2+, a negative in situ hybridization (FISH, CISH, or SISH) test is required to confirm the HER2-negative status (based on the most recently analyzed tissue sample tested by a local laboratory) 4. Histologically confirmed ER-positive and/ or PgR-positive breast cancer determined by core biopsy according to local in-house standard 5. Patient has already initiated or is about to initiate first-line therapy with a CDK4/6 inhibitor (any CDK4/6i) in combination with an aromatase inhibitor (non-steroidal AI). Treatment must be intended as first-line systemic therapy for advanced or metastatic disease 6. Information about prior oncological therapies against the HR+/HER2- BC must be available 7. Data about histo-pathological report must be available, including hormone receptor status, tumor histology, tumor grading and HER2 immunohistochemistry 8. Inclusion in the study occurs as a single, one-time event for each patient, within one of the 9 defined cohorts Exclusion Criteria: 1. Patients with an interruption of ongoing first-line CDK4/6i plus AI therapy exceeding 1 month for either CDK4/6i or AI 2. Patients receiving any other systemic anticancer therapy for HR+/HER2- aBC, except for LHRH agonists, bisphosphonates, or denosumab 3. Clinical evidence of progression at the timepoint of study inclusion 4. Patients have already completed ≥ 1L of systemic therapy for aBC