A prior study indicated that caspase recruitment domain family member 9 knockout (CARD9−/−) mice had increased susceptibility to colitis, causing altered gut microbiome and damaged Trp metabolism (28), while AhR activation by supplementation with 6-formylindolo (3,2-b) carbazole (Ficz), a typical Trp-derived AhR agonist (29), alleviated colitis scores in CARD9−/− mice (28). This evidence concerns the gene CARD9 and colitis.