Researchers at the National Cancer Institute are studying the efficacy of immunotherapy for people with metastatic breast cancer. Immunotherapy involves using a person’s immune cells to fight tumors, such as breast cancers. Many patients with metastatic breast cancer can use their tumor-infiltrating lymphocytes (TILs), which are found within and around the tumors, to create an immune response. TILs work by recognizing the proteins on the surface of tumor cells, called neoantigens and neoantigens are created from mutations that are found in tumor DNA.