Indoleamine 2,3-dioxygenase 1 (IDO1) is a heme-containing enzyme that catalyzes the breakdown of tryptophan (trp), which is essential for immune function, to kynurenine (kyn), thereby inhibiting CD8+ T cell activity and up-regulating immunosuppressive Treg, resulting in the inability of the immune system to respond appropriately to cancer cells, and thus IDO1 is considered an important target for cancer immunotherapy (Fig. 4A).75 The gene discussed is CD8A; the disease is cancer.