The expression levels of most Trp metabolism-related genes, such as OGDHL, ALDH1B1, CYP1A2, GCDH, ACAT1, TDO2, ALDH2, HADH, ALDH9A1, KYNU, KMO, CYP1A1, INMT, MAOA, ECHS1, IDO2, ACMSD, AOX1, EHHADH, MAOB, HAAO, AOC1, AADAT, ACAT2, and CAT, were downregulated in HCC tissues than in paracarcinoma tissues (Fig. 1A). Here, HAAO is linked to hepatocellular carcinoma.