IDO1 and neoplasm: For example, IDO (indoleamine 2,4-dioxygenase), one of the top EBV-induced immune inhibitors, may inhibit the activities of cytotoxic T lymphocytes and NK cells by causing local tryptophan depletion in the tumor niche and thus help promote tumor survival [28,73,74,75], despite the enhanced immune infiltration in the EBV-high NSCLC.