Rigorous eligibility criteria based on specific co-morbidities and clinicopathologic features of their disease have been designed to minimize the risk of patients participating in this study. The anticipated favorable clinical benefits of elacestrant combined with everolimus are projected to outweigh the risks of this treatment. This study will be performed in full compliance with International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) and all applicable local Good Clinical Practice (GCP) and regulations. conditions: Advanced Breast Cancer conditions: ER-positive Breast Cancer conditions: HER2-negative Breast Cancer conditions: ESR1 Gene Mutation studyType: INTERVENTIONAL phases: PHASE3 allocation: RANDOMIZED interventionModel: PARALLEL interventionModelDescription: Double-blind, parallel group (two arms: elacestrant plus everolimus, and elacestrant plus placebo). primaryPurpose: TREATMENT masking: DOUBLE maskingDescription: A blinded, independent IRC will perform a review of radiographic images and clinical information collected on study to determine the protocol-defined endpoints of disease response and progression. Further information on the independent review process will be provided in the BIRC Charter. whoMasked: PARTICIPANT whoMasked: INVESTIGATOR count: 240 type: ESTIMATED name: Everolimus name: Elacestrant name: Placebo name: Auxiliary Medicinal Product - Dexamethasone name: Auxiliary Medicinal Product - Luteinizing hormone-releasing hormone (LHRH) analogues measure: To demonstrate superiority of elacestrant+everolimus vs. elacestrant+placebo in prolonging PFS based on a BIRC in patients with ER[+]/HER2[-], ESR1-mutated, ABC that have previously received ET+CDK4/6i (all patients). measure: To compare overall survival (OS) between treatment groups, in all patients. measure: To compare investigator-assessed PFS based on local assessment between treatment groups, in all patients. measure: To compare objective response rate (ORR) between treatment groups, in all patients. measure: To compare clinical benefit rate (CBR) between treatment groups, in all patients. measure: To compare overall time to response (TTR) between treatment groups, in all patients. measure: To compare duration of response (DoR) between treatment groups, in all patients. measure: To compare best percentage of change in tumor burden between treatment groups, in all patients. measure: To describe the changes in health-related quality-of-life (HRQoL) from baseline using the EuroQoL 5 Dimension 5 Level (EQ-5D-5L) scale. measure: To describe the changes in HRQoL from baseline using the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 (EORTC QLQ-C30). measure: To describe the changes in HRQoL from baseline using the EORTC Quality of Life Questionnaire Breast Cancer module (EORTC QLQ-BR45). measure: To assess adverse events (AEs) between treatment groups in all patients. measure: To assess adverse events leading to study treatment withdrawal. measure: To assess adverse events leading to dose modification. measure: To assess the number of participants with abnormal hematology and/or chemistry values that are related to treatment. measure: To assess the number of participants with changes on vital signs and/or AEs that are related to treatment. measure: To assess the number of participants with changes on cardiac function measured by ECG QT Interval Corrected by Fridericia formula (QTcF) that are related to treatment effect . measure: To assess the number of participants with changes on Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) that are related to treatment. measure: Exploratory endpoint: protein expression studies using blood samples to investigate the potential association with clinical outcomes. measure: Exploratory endpoint: analysis of allele mutation frequencies in circulating tumor DNA (ctDNA) in blood samples to investigate the potential association with clinical outcomes. measure: Exploratory endpoint: Genetic studies using blood samples to investigate the potential association with clinical outcomes. measure: Exploratory endpoint: Analysis of medical imaging (radiomics) to identify potential biomarkers associated with clinical outcomes. measure: Exploratory endpoint: Pharmacokinetic (PK) parameter of maximum (Peak) Observed Serum Concentration (Cmax). measure: Exploratory endpoint: Pharmacokinetic (PK) parameter of Plasma Through Concentration (Ctrough). sex: ALL minimumAge: 18 Years stdAges: ADULT stdAges: OLDER_ADULT hasResults: False