The evaluation of the clinical efficacy in metastatic melanoma patients of the inhibition of IDO in combination with the antagonistic antibody targeting the immunoregulatory molecule programmed cell death-1 (PD-1) resulted in high grade of toxicity and the anticipated closure of the Phase III clinical study, indicating that further investigations on the mechanisms of action of IDO inhibitors and their synergy with other therapeutic interventions need to be performed. The gene discussed is IDO1; the disease is metastatic melanoma.