Although phase-I/II trials of the IDO inhibitor-plus-pembrolizumab in solid tumors, including melanoma, showed promising results, a phase-III trial in patients with unresectable or metastatic melanoma failed due to not meeting the primary end-point [109,110], and no significant difference between the combination treatment and pembrolizumab-alone groups was observed [110]. This evidence concerns the gene IDO1 and melanoma.