IDO1 and cancer: In mouse model of melanoma, cancer cells develops resistance to anti-CTLA4 therapy by the upregulation of IDO1, and CTLA-4 antibody strongly synergized with IDO1 inhibitors to mediate elimination of IDO1-positive and negative immunogenic tumors in a T cell dependent manner, underlining the importance of the inhibitory role of both tumor- and host-associated IDO1 (73).