Given the central role of CYP3A4 to the metabolism of over 50% of drugs in clinical use, including the corticosteroids prednisone and budesonide, as well as newer targeted agents such as tofacitinib used to treat IBD, our findings add important new insights on the interplay between bile acid composition in CD and nuclear receptors of direct relevance to optimal dosing of CYP3A4 substrate drugs in patients with CD. The gene discussed is CYP3A4; the disease is inflammatory bowel disease.