24-hydroxycholesterol is transported from the endoplasmic reticulum to the extracellular space. In humans, this event is the major source of 24-hydroxycholesterol in the blood and is the means by which the molecule, generated from cholesterol in the brain, is transported to the liver for conversion to bile acids and bile salts. While transport proteins are likely to play a role in this process, and the 24-hydroxycholesterol is likely to occur as part of a lipoprotein complex in the blood, the relevant proteins have not been identified (Lutjohann et al. 1996; Bjorkhem et al. 1998).