IDO1 and neoplasm: It is also notable that other studies indicate the potential role of Kyn on Treg activation as evidenced by the Munn and Mellor group who showed IDO-expressing plasmacytoid DCs in tumor draining lymph nodes preferentially activate pre-existing Treg through the PD-1-PTEN pathway which was inhibited by the IDO pathway non-enzyme inhibitor, 1-methyl-D-tryptophan (1-MT) (63, 64).