IDO1 and neoplasm: The IDO1 is a heme enzyme that catabolizes tryptophan (Trp) into kynurenine, while IDO catalyzes oxidative catabolism of tryptophan. The Trp metabolite production and Trp depletion in TME lead to inhibit T‐cell responses, including increased T‐cell apoptosis, naive T cells differentiation into T regulatory cells, and reduced T‐cell proliferation. Consequently, tumor‐specific T‐cell response could be inhibited by IDO expression, and IDO inhibition can improve T‐cell therapy for cancers.