IDO1 and neoplasm: 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).