**Study of XMT-2056 in Advanced/Recurrent Solid Tumors That Express HER2** This study is testing a new treatment called XMT-2056 for people with advanced or returning solid tumors that show a protein called HER2. The study is in its first phase, which means it's the first time XMT-2056 is being tried in humans. The goal is to find a safe dose and see how well it works against tumors. **HER2** is a protein that can help cancer cells grow. If your tumor shows HER2, it may be suitable for this study. Participants must have tried other treatments without success or cannot take them due to side effects. They need to provide a sample of their tumor for testing. People with certain medical conditions or who are on specific medications might not qualify. **Key Points:** - The study involves multiple visits to monitor safety and effects of the treatment. - Participants may need to provide fresh or archival tumor samples for testing. - People with untreated brain metastases cannot join unless their condition is stable for at least 2 weeks.