24-hydroxycholesterol is transported from the extracellular space to the endoplasmic reticulum. In humans, this event is the means by which the molecule, generated from cholesterol in the brain, is taken up by liver cells for conversion to bile acids and bile salts. While transport proteins are likely to play a role in this process, these proteins have not been identified (Lutjohann et al. 1996; Bjorkhem et al. 1998).