IDO1 and cancer: Indoleamine-2,3-dioxygenase 1 (IDO1), a rate-limiting tryptophan (Trp)-degrading enzyme, has been intensively pursued as a drug target to reverse the immunosuppressive cancer-immune milieu and promote tumor elimination.1 2 As an important immunoregulator, IDO1 not only triggers immunosuppressive mechanisms but also orchestrates the immune equilibrium of individuals.3 4 Resembling programmed death-ligand 1 (PD-L1), the expression and activity of IDO1 in the immune compartment is tightly regulated.