We found that the enzymes IDO1 and TDO2 were elevated in ∼40% of the samples from colon cancer patients, and the enzyme AFMID, which is involved in the last step of the conversion of Trp into Kyn, was up-regulated in 80% of these samples of colon cancers (Fig. 3B). Here, TDO2 is linked to malignant colon neoplasm.