IDO2 is a presumptive immunomodulatory gene based on its close structural relationship to IDO1 and its expression in a variety of antigen-presenting cell types. Both IDO1 and IDO2 will catabolize tryptophan to kynurenine. Biochemical studies indicate that both enzymes are similarly robust in catabolic activity, although the in vitro conditions required for IDO2 to manifest the same level of activity differ somewhat from IDO1. However, whether IDO2 is active as a tryptophan catabolizing enzyme in human dendritic cells has been disputed. Further work is needed to conclusively determine that IDO1 and IDO2 are similar in their preference for substrates and reaction pathways to generate product. A non-redundant function for IDO2 relative to IDO1 is suggested by their rather distinct expression patterns and response to extracellular stimuli.