Brain metastases are the most common type of brain tumors, affecting nearly 200,000 people in the United States each year. Once diagnosed, patients have a median survival of sic months. Immunotherapy, a type of cancer treatment that helps your immune system fight cancer, is on the rise and have already shown potential in several cancers. Yet, brain metastases are known to possess extraordinary genetic variability and special features that allow them to impede immunotherapy.