IDO1 and neoplasm: IDO-1, the rate-limiting enzyme involved in the conversion of the essential amino acid tryptophan to its catabolic product kynurenine (Kyn) (204, 243), can be upregulated in DCs, by Tregs as mentioned above, as well as by tumor cells and myeloid cells in response to IFNγ (244, 245), and has been associated with poor prognosis for several types of cancer including ovarian (246), endometrial (247), colorectal (248), and lung (249).