Unfortunately, the combined use of epacadostat and pembrolizumab did not improve progression-free survival of the melanoma patients in a phase III trial.23 These observations suggest that patient stratification is important for IDO1 inhibitor-based treatment, and elucidation of the crosstalk between IDO1 and other pathways and the mechanisms underlying IDO1 constitutive activation in cancers will be critical to development of effective therapeutic strategies. This evidence concerns the gene IDO1 and melanoma.