DiviTum®TKa: A Biomarker Assay for Efficacy in HR+ Breast Cancer Patients

This is an observational study (no experimental treatment) that tracks a blood-based test called DiviTum TKa to see if it can predict how well hormone-receptor-positive (HR+) metastatic breast cancer responds to CDK4/6 inhibitor drugs (like palbociclib, ribociclib, or abemaciclib). **You may be eligible if...** - You are a woman (pre- or post-menopausal) with HR+ metastatic breast cancer - You are starting or already receiving a CDK4/6 inhibitor drug with or without hormone therapy, OR you are starting a hormone therapy alone for metastatic disease **You may NOT be eligible if...** - You are using an experimental/investigational treatment - You have early-stage (not metastatic) breast cancer - You are male - You are being actively treated for another cancer Talk to your doctor to see if this trial is right for you.