Cerebrotendinous xanthomatosis is characterized by the accumulation of cholestanol and cholesterol in xanthomas and brain causing a number of severe symptoms. More than 20 different mutations have been identified in the gene encoding sterol 27-hydroxylase. Defects in the gene lead to reduced bile acid biosynthesis, with accumulation of 7 alpha-hydroxylated intermediates, one of which is a precursor to cholestanol. The disease can be treated successfully with chenodeoxycholic acid, which reduces the upregulation of cholesterol 7 alpha-hydroxylase and, therefore, the formation of cholestanol. Disruption of the gene encoding sterol 27-hydroxylase in mice does not have the same metabolic consequences as in humans.