RecruitingPhase 2Drug trialLiquid-biopsy Informed Platform Trial to Evaluate CDK4/6-inhibitor Resistant ER+/HER2- Metastatic Breast CancerThis study is being done to answer the following question: Can testing breast cancer for DNA abnormalities or "biomarkers" help predict which patients are most likely to be helped by certain treatments? The pre-study screening is being done to test a sample of blood (or tumour tissue) for biomarkers to see if patients can participate in the study