(a) The classical pathway for bile acid synthesis begins with CYP7A1, which converts cholesterol into 7α-hydroxycholesterol. This pathway mainly produces cholic acid in humans. (b) Alternate pathways for bile acid synthesis. Cholesterol is first converted into oxysterols by one of three different enzymes: sterol 27-hydroxylase (CYP27), expressed in multiple tissues including liver; cholesterol 25-hydroxylase, present at low levels in multiple tissues including heart, lung, and kidney; and cholesterol 24-hydroxylase (CYP46), expressed predominantly in the brain. Oxysterols are transported through the bloodstream to the liver, where they are 7α-hydroxylated by oxysterol 7α-hydroxylase (CYP7B1) in the case of 25- and 27-hydroxycholesterol and by CYP39A1 in the case of 24-hydroxycholesterol. Alternate pathways preferentially produce chenodeoxycholic acid in humans.