Holmgaard et al. (76) demonstrated that when the IDO inhibitor 1-methyltryptophan (1MT) was combined with CTLA-4 treatment in mice with B16 melanoma that was resistant to CTLA-4 treatment, the tumors showed treatment sensitivity, suggesting that IDO inhibition could be a potential adjuvant therapeutic strategy for immunotherapies to reduce the development of resistance (76). This evidence concerns the gene IDO1 and melanoma.