AHR and inflammatory bowel disease: A recent study has found a relationship between gut microbiota and caspase recruitment domain family member 9 (CARD9), a susceptibility gene for IBD involved in recovery from colitis and shown that the microbiota from CARD9-deficient mice failed to convert tryptophan into indole derivatives that activate AHR, the reduced production of which was also observed in the microbiota from human subjects with IBD [257].