TDO2 and glioblastoma: Additionally, recently described overexpression of indoleamine 2,3-dioxgynase 1 and tryptophan 2,3-dioxygenase 2 (IDO1/TDO2) in GBM promotes immunosuppression through the production of tryptophan metabolite L-Kynurenine (Kyn) [56].