We have for many years been interested in the enzymes that convert cholesterol, an otherwise hydrophobic and insoluble molecule, into bile acids, which are water-soluble derivatives that are readily excreted from the body (Figure 1). Bile acids are made in the liver by metabolic pathways that include at least 16 enzymes located in the cytoplasmic and membrane compartments of the cell (Illustration 2). We use biochemical and molecular methods to isolate these enzymes and their encoding genes and then apply physiological methods and targeted gene disruption in mice to determine their roles in metabolism. For example, the liver-specific enzyme 3b-hydroxy-Δ5-C 27-steroid oxidoreductase (3b-HSD) is located in the endoplasmic reticulum and catalyzes an early step in the synthesis of bile acids.