A steroid is an organic compound with four fused rings (designated A, B, C, and D) arranged in a specific molecular configuration.
Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and as signaling molecules. Examples include the lipid cholesterol, sex hormones estradiol and testosterone, anabolic steroids, and the anti-inflammatory corticosteroid drug dexamethasone. Hundreds of steroids are found in fungi, plants, and animals. All steroids are manufactured in cells from a sterol: cholesterol (animals), lanosterol (opisthokonts), or cycloartenol (plants). All three of these molecules are produced via cyclization of the triterpene squalene.