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, ALDH1B1 is linked to hepatocellular carcinoma.