Although significant progress has been made in the last decade demonstrating the relevance of a non-enzymatic activity of IDO1 in both immune and tumor cells (7, 14, 18), several issues remain to be addressed, including i) to characterize the molecular partners involved in IDO1 signaling in different tumor types in order to optimize synergistic therapies and establish more reliable pharmacodynamic markers; ii) to demonstrate a tumor-specificity of IDO1 signaling. The gene discussed is IDO1; the disease is neoplasm.