PDCD1 and cancer: IDO1 is a tryptophan-catabolizing enzyme that isoverexpressed in many cancers and induces immune tolerance by suppressing T-cellresponses; IDO1 inhibition exhibits antitumor activity through the reactivation ofeffector T cells and is synergistic with PD-1 blockade [78–83] Suchcombination is currently being investigated in a randomized, placebo-controlled phaseIII clinical trial (NCT02752074) [78].