To characterize the Trp metabolic pathway in colon cancer tissues, expressions of Trp transporters (SLC1A5 and SLC7A5) and Kyn metabolism-related enzymes (IDO1 and AFMID) as well as serotonin metabolism-related enzymes (TPH1, MAOA, MAOB and TG2) were analyzed in a colon cancer tissue microarray (TMA) containing 66 matched pairs of carcinoma and adjacent tissue samples by immunohistochemistry (IHC). The gene discussed is MAOB; the disease is carcinoma.