Cholesterol is an essential archetypical sterol of cell membranes throughout all organisms. The metabolism of cholesterol starts from its oxidation to oxysterols. The precursors of cholesterol can also be oxidized to oxysterols and be metabolized. Therefore, oxysterols are the oxidized forms of cholesterol molecules, with one or more additional carbonyl, hydroxyl or carboxylic acid groups in the ring or the side-chain of the cholesterol skeleton.