These new evidences thus warrants further investigation to delineate the mechanism of action underlying the impact of GDC-0919 on NK cell activity toward tumor cells and calls the interest to analyze NK cell features/biology in cancer patients undergoing treatment regimen combining immune checkpoint blockers (ICB) with IDO inhibitors—all of these to give insights in order to design novel therapeutic strategies improving ICB efficacy. Here, IDO1 is linked to neoplasm.