However, it is unlikely that the other two tryptophan-degrading enzymes, IDO-2 and tryptophan 2,3-dioxygenase (TDO), were responsible for such phenomenon because the former appears to be expressed as an inactive form in human dendritic cells [52] and tumor cells [53], while the latter is exclusively expressed by hepatocytes [54]. The gene discussed is IDO2; the disease is neoplasm.