Cholestnol is a physiological sterol in animals, but occurs in far lower concentrations than does cholesterol. The chemical structure of cholestanol resembles that of cholesterol, missing only a double bond at the 5-6 position of the B-ring. The large difference in these concentrations has made it difficult to investigate the role of cholestanol in the body. The accumulation of cholestanol was reported by Menkes in CTX patients, which suggested a pathological role of cholestanol in reverse of its physiological function.