The limited number of T lymphocytes in GBM, which is isolated by the blood–brain barrier, means that immunotherapies targeting T cells such as monoclonal antibodies against programmed cell death 1 (PD-1) or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) are not suitable for treating GBM (46). The gene discussed is CTLA4; the disease is glioblastoma.