For example, in cerebrotendinous xanthomatosis, when the normal route of side‐chain oxidation is blocked by sterol 27‐hydroxylase deficiency, CA can be produced by microsomal hydroxylation at C24 and C25 (catalysed by CYP3A4 [7]) followed by cleavage between these carbon atoms [8]. The gene discussed is CYP27A1; the disease is hyperinsulinemic hypoglycemia, familial, 4.