Figure 1. Bile acids are synthesized from cholesterol in the liver through two pathways: the classic pathway and the alternative pathway. The bile acid synthesis mainly produces two primary bile acids, cholic acid and chenodeoxycholic acid (CDCA). In the classic pathway, the mitochondrial sterol 27-hydroxylase (CYP27A1) catalyzes the steroid side-chain oxidation in both CA and CDCA synthesis. In the alternative pathway, CYP27A1 catalyzes the first step to convert cholesterol to 27-hydroxycholesterol which eventually is converted to CDCA.