Gene expression levels of ABCB4, ABCB11 and ABCG5/8 are regulated by at least two nuclear recep- tors (NR) initially found to regulate cholesterol and bile acid metabolism [52, 53]. The farnesoid or bile acid receptor FXR/BAR [54–57] regulates transcription of ABCB4 [58] and ABCB11 [59] while ABCG5 and ABCG8 are under control of the oxysterol or liver X receptors LXRa/b [60], perhaps mediated by FXR [61].