This study aims to understand how well a drug called **trastuzumab deruxtecan (T-DXd)** works for adults with a specific kind of breast cancer. This breast cancer is called **HER2-negative**, meaning it shows low or no activity of a gene known as HER2, which is often involved in breast cancer. Here's how T-DXd works: it combines a special protein (monoclonal antibody) with a cancer-fighting agent. This combination seeks out cancer cells and disrupts their DNA, helping to stop their growth while leaving healthy cells unharmed. Researchers want to see if T-DXd can help people with HER2-negative cancer, which typically wouldn’t benefit from other HER2-targeted therapies. **Key Points:** - **Duration & Visits:** Each treatment cycle lasts 21 days, and you may need multiple cycles. - **Eligibility:** You must be 18 or older with specific types of breast cancer that have spread and meet other health criteria. - **Risks and Benefits:** The study aims to offer potential benefits for specific patients, but risks and side effects are possible. Discuss with your doctor to understand if it's right for you. If you meet the criteria and are interested, consider joining to help advance cancer treatments.