Humans and mice have substantially different bile acid (BA) pool compositions (1). As the major primary BAs, humans synthesize cholic acid (CA) and chenodeoxycholic acid (CDCA), whereas mice have mainly CA and 6-hydroxylated muricholic acids (MCAs) that are made from CDCA (Fig. 1). Hydroxylation at the C-6 position significantly affects the physicochemical properties of BAs, making the BA pool more hydrophilic, less potent as detergents, and less injurious. In addition, 6-hydroxylation dramatically changes BA signaling properties, converting the most potent endogenous FXR agonist (CDCA) to antagonists (MCAs).