IDO1 and cancer: In addition to PD1 and PD-L1, Indoleamine 2,3-dioxygenase-1 and -2 (IDO1,2) an intracellular enzyme overexpressed by many cancers, have also been shown to be involved in immune suppression via the depletion of tryptophan from the microenvironment and the production of metabolites, such as kynurenines [42].