Biomarkers and Clinical Features of Metastatic Breast Cancer in Patients Treated With CDK4/6 Inhibitors

This study is collecting blood and tissue samples from patients with hormone receptor-positive (ER+), HER2-negative metastatic breast cancer who are being treated with CDK4/6 inhibitors (such as palbociclib, ribociclib, or abemaciclib) to find biomarkers that predict who responds best to these drugs. **You may be eligible if...** - You are an adult with ER+/HER2- metastatic breast cancer or high-risk early breast cancer - You are currently being treated with, or have been treated with, a CDK4/6 inhibitor (a class of targeted therapy drugs) - You are receiving this as standard of care or as part of another eligible study **You may NOT be eligible if...** - You have a different subtype of breast cancer (e.g., HER2-positive or triple-negative) - You have not received or are not eligible for CDK4/6 inhibitor therapy Talk to your doctor to see if this trial is right for you.