Metabolic syndrome is reported to be associated with impaired AhR agonist production by gut microbiota-dependent tryptophan metabolism, and treatment with an AhR agonist (Ficz, 6-formylindolo (3,2-b) carbazole) or engineered Lactobacillus reuteri strain with superior tryptophan metabolizing capacities can relieve metabolic disorders and liver steatosis in HFD-fed mouse models, primarily by inducing intestinal IL-22 production and cytochrome P450 1A1 (CYP1A1) gene expression [89]. The gene discussed is CYP1A1; the disease is fatty liver disease.