IDO1 and neoplasm: With the use of immunodeficient mouse models, Ninomiya et al. [11] showed that tumor-expressing indoleamine 2,3-dioxygenase (IDO), an intracellular enzyme that has an inhibitory effect on T cells, can be better controlled by combining CAR-T cell therapy with 1-methyl-tryptophan (1-MT), an IDO inhibitor.