CYP27A1 and cerebrotendinous xanthomatosis: One example is cerebrotendinous xanthomatosis (CTX) where the enzyme CYP27A1 is deficient resulting in reduced levels of its product (25R)26-hydroxycholesterol ((25R)26-HC) and greatly elevated levels of 7α-HC and 7α-HCO in plasma (Fig. 2C and D) (DeBarber et al., 2011).