l-kynurenine, a tryptophan-derived catabolite resulting from the activity of indoleamine 2,3-dioxygenase (IDO), an enzyme that is often expressed in tumor cells or tumor-associated cells (TAMs, dendritic cells, etc.)(81, 82), interferes with NK cells by regulating the surface expression of activating receptors and thus their antitumor function (83). Here, IDO1 is linked to neoplasm.