Elacestrant in Patients With ER+ HER2- ESR1-mutated Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria: * Signed and dated informed consent form * Postmenopausal women * Age ≥18 years * Eastern Cooperative Oncology Group Performance Status (ECOG) \< 2 * Locally advanced and/or metastatic ER+ HER2- breast cancer * Histologically proven ER positivity (defined as ≥1% staining by immunohistochemistry (IHC)) * Histologically proven HER2 negativity (defined as a IHC0 or IHC1+ score by IHC or a negative result by in situ hybridization (ISH), optionally combined with a IHC2+ score) * Disease progression following first line ET + CDKi * No more than one prior ET line in the advanced/metastatic setting and intention for 2nd-line treatment with elacestrant according to current elacestrant SmPC as assessed by the treating physician (ESR1 testing can be done after inclusion) * For patients with proven ESR1mut: Study inclusion the latest 2 weeks after start of elacestrant treatment Exclusion Criteria * Prior chemotherapy in the advanced/metastatic setting * Contraindications according to elacestrant SmPC, except for ESR1 test result for patients included prior to ESR1 testing. * Participation in an interventional clinical trial within 30 days prior to enrolment or simultaneous participation in an interventional clinical trial (except follow-up phase)