Indoleamine 2,3-dioxygenase 1 (IDO-1), an enzyme critical in tryptophan-to-kynurenine conversion, plays a pivotal role in modulating T-cell mediated immunity via aryl hydrocarbon receptor activation, a process integral to immune evasion in cancers such as glioblastoma, and correlates with poor prognosis [152, 168, 169]. This evidence concerns the gene IDO1 and cancer.