CYP27A1, a mitochondrial matrix sterol hydroxylase, catalyses the 27-hydroxylation of side-chains of sterol intermediates (Cali et al. 1991). In the bile acid synthesis pathway, CYP27A1 catalyses the first step in the oxidation of the side chain of sterol intermediates such as cholestane-triols (Pikuleva et al. 1998). Defects in CYP27A1 can cause Cerebrotendinous xanthomatosis (CTX; MIM:213700), a rare sterol storage disorder. Decreased bile acid production results in the accumulation of sterol intermediates in many tissues, including brain. The disorder is characterised by progressive neurologic dysfunction, premature atherosclerosis and cataracts (Gallus et al. 2006).