IDO1 and neoplasm: Conversely, several studies have been reported regarding the inhibition of immunosuppressive targets, such as VISTA (v-domain Ig suppressor of T-cell activation), a ligand, and IDO1 (indoleamine 2,3-dioxygenase-1), an enzyme catalyzing the kynurenine pathway as a rate-limiting step for enhancing anti-tumor T-cell responses and inhibition of immune responses by depletion of tryptophan [37,38].