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). The gene discussed is ALDH9A1; the disease is hepatocellular carcinoma.