NR1H4 and cystic fibrosis: Although the expression levels of Na/Taurocholate Cotransporting Polypeptide (NTCP), a major transporter of hepatic BA (33), and FXR, a transcriptional regulator of BA synthesis and transport (34, 35), were not significantly different between the WT and CF rabbit livers, the expression levels of the transcripts encoding Cytochrome P450 Family 7 Subfamily A Member 1 (CYP7A1), the rate-limiting enzyme mediating BA biosynthesis and metabolism (36), in the CF rabbit livers was significantly reduced, compared to that of the WT rabbit livers (Fig. 2E).