The indoleamine 2,3-dioxygenase 1 (IDO-1) is a checkpoint molecule found on GBM and immune cells, which can convert tryptophan to N-formyl kynurenine (NFK) and its eventual downstream catabolite kynurenine (KYN) by formamidase, and has since been implicated in immunosuppression [67]. The gene discussed is IDO1; the disease is glioblastoma.