The addition of a 7‐hydroxyl group is an early and often rate‐limiting step in the synthesis of bile acids. This reaction is catalysed by two cytochrome P450 enzymes known as cholesterol 7α‐hydroxylase and oxysterol 7α‐hydroxylase. cDNAs encoding these proteins have been isolated and used to define two evolutionarily conserved pathways that produce 7α‐hydroxylated bile acids. Curr Opin Lipidol 9:113–118. © 1998 Rapid Science Ltd