IDO1 and cancer: Both indoleamine 2,3-deoxygenase 1 (IDO1) and programmed death 1 (PD-1) mediated pathways suppress T-cell–mediated antitumor immunity, and IDO1 and the PD-1 ligand (PD-L1) are co-expressed in multiple cancer types and correlate with poor prognosis [9–15].