This model reflects many other observed changes described in limited studies in humans. For example, in humans, the activity of HMG-CoA reductase and CYP7a1 have been reported to be low [7,39]. In this study, we show that the mRNA and enzyme activities in the livers from knockout animals are also significantly reduced. Additionally, the activity of the rate-limiting enzyme for bile acid synthesis, CYP7a1, is reduced, although no changes at the mRNA level are noted. This is also in keeping with previous studies that show that sitosterol is a direct inhibitor of this enzyme in vitro [39].