CYP7A1 and Smith-Lemli-Opitz syndrome: That view has changed with the discovery that 7-OC can be generated enzymatically from the cholesterol precursor 7-dehydrocholesterol (7-DHC) by cytochrome P450 (CYP) 7A1 (5) and is abundant in plasma of patients with Smith-Lemli-Opitz Syndrome (SLOS), where levels of 7-DHC are high, and cerebrotendinous xanthomatosis, where CYP7A1 is highly expressed (6, 7).