Indoleamine 2,3-dioxygenase (IDO), a tryptophan-metabolizing enzyme, plays a key role in glioma immune evasion by depleting tryptophan, essential for T-cell function, and promoting Treg infiltration (68, 69), This dual mechanism suppresses effector T-cell activity and facilitates glioma progression, with studies linking higher IDO expression to increased tumor malignancy and worse prognosis (69, 70). This evidence concerns the gene IDO1 and central nervous system cancer.