Biomarkers to Detect Endocrine Therapy Resistance

This study collects blood samples over time from patients with ER-positive, HER2-negative advanced or metastatic breast cancer to find biological markers (biomarkers) that can predict whether hormone therapy will stop working — a process called endocrine resistance. The goal is to detect resistance earlier and guide future treatment decisions. **You may be eligible if...** - You are 18 or older - You have ER-positive (≥1%), HER2-negative advanced or metastatic breast cancer (locally advanced, or "lobular breast cancer" subtype also eligible) - You are starting a new standard hormone therapy - You have evaluable disease (measurable tumor or bone-only disease) - Your organ function is adequate for systemic treatment **You may NOT be eligible if...** - Your cancer is ER-negative or HER2-positive - You are not starting a new hormone therapy - Your tumors cannot be evaluated by imaging or lab markers Talk to your doctor to see if this trial is right for you.