Analysis of Circulating Tumor DNA (ctDNA) in HER2-low Advanced/Metastatic Breast Cancer

This study is collecting blood samples from patients with HER2-low advanced or metastatic breast cancer to analyze circulating tumor DNA (ctDNA) — tiny fragments of cancer DNA that float in the bloodstream — before treatment begins, in order to better understand the disease and track how it responds to therapy. **You may be eligible if...** - You have been diagnosed with HER2-low breast cancer (meaning your cancer has a low level of the HER2 protein, confirmed by lab testing) - Your cancer is advanced or has spread to other parts of the body - You have not yet started treatment - You are accessible for follow-up visits **You may NOT be eligible if...** - You have already started treatment for advanced or metastatic disease - You are unable or unwilling to provide written consent Talk to your doctor to see if this trial is right for you.