Expression of CYP7A1 in wild-type and knockout mice. (a–c) CYP7A1 mRNA, enzyme activity (expressed as pmol product per minute per mg microsomal protein), and protein were assessed by blotting, mass spectrometry, and immunoblotting, respectively, in male and/or female mice (n = 6 per group) of the indicated genotypes. Cyp27a1–/– mice have a reduced bile acid pool size and synthetic rate due to an induced mutation in the gene for sterol 27-hydroxylase, an enzyme involved in the synthesis of all primary bile acids. Cyp7a1–/– mice served as negative controls in these experiments. BIP, immunoglobulin heavy-chain binding protein.