In CTX, caused by deficiency in the enzyme CYP27A1, essential to the acidic pathway of bile acid biosynthesis, CYP7A1 is up-regulated as a consequence of reduced negative-feedback by primary bile acids and it is probable that 7-OC is formed enzymatically by up-regulated CYP7A1 using 7-DHC as a substrate. This evidence concerns the gene CYP7A1 and cerebrotendinous xanthomatosis.