CYP27A1, a mitochondrial matrix sterol hydroxylase, catalyses the 27-hydroxylation of side-chains of sterol intermediates (Cali et al. 1991). A specific reaction is the 27-hydroxylation of 5beta-cholestan-3alpha, 7alpha, 12alpha-triol (5bCHOL3a,7a,12a-triol) to form 5beta-cholestan-3alpha, 7alpha, 12alpha, 27-tetrol (5bCHOL3a,7a,12a, 27-tetrol) (Pikuleva et al. 1998). Defects in CYP27A1 can cause Cerebrotendinous xanthomatosis (CTX; MIM:213700), a rare sterol storage disorder characterised by progressive neurologic dysfunction, premature atherosclerosis and cataracts (Cali et al. 1991b).