Interestingly, in the context of gliomas, it has been observed that the presence of an isocitrate dehydrogenase (IDH) mutation is associated with reduced expression of immunological checkpoint molecules, including PD-1, cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), lymphocyte-activation gene 3 (LAG3), and indoleamine 2,3-dioxygenase 1 (IDO1) [58]. Here, IDO1 is linked to glioma.