Hydroxycholesterols are mono-oxygenated derivatives of cholesterol that comprise a family of polycyclic compounds that contain a second oxygen atom as a hydroxyl group on the skeleton of cholesterol. In vivo, hydroxycholesterols are present as unesterified (biologically active) and esterified forms (largely biologically inert). Hydroxycholesterols are formed through enzymatic conversion of cholesterol or by free radical autoxidation, and exhibit a short half life relative to cholesterol.