A P450 heme-thiolate protein that is found in liver microsomes and in ciliary non-pigmented epithelium [2]. The enzyme is specific for (24S)-cholest-5-ene-3¦Â,24-diol, which is formed mostly in the brain by EC 1.14.14.25, cholesterol 24-hydroxylase. The direct electron donor to the enzyme is EC 1.6.2.4, NADPH¡ªhemoprotein reductase.