Genetically-informed Therapy for ER+ Breast Cancer Post-CDK4/6 Inhibitor

This study is looking at the best treatment options for postmenopausal women with hormone receptor-positive (ER+) breast cancer that has spread, after their cancer stopped responding to a class of drugs called CDK4/6 inhibitors (such as palbociclib, ribociclib, or abemaciclib). It uses genetic information from tumor samples to guide which treatment is best. **You may be eligible if...** - You are a postmenopausal woman, 18 or older - You have ER+ (hormone receptor-positive) breast cancer that has spread or cannot be cured - You previously received a CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) for advanced disease - You have had no more than 3 additional lines of therapy since the CDK4/6 inhibitor, and no more than 1 prior chemotherapy line - A recent biopsy of your cancer is available or obtainable **You may NOT be eligible if...** - Your cancer does not express estrogen receptors (ER-) - You have not yet received a CDK4/6 inhibitor - You cannot provide a tissue sample for genetic testing - Your general health or organ function does not meet study requirements Talk to your doctor to see if this trial is right for you.