Steroid skeleton of lanosterol. The total number of carbons (30) reflects its triterpenoid origin. In some steroids some carbons may be removed (such as carbon 18) or added (such as carbons 241 and 242) in downstream biosynthetic reactions. A steroid is a terpenoid lipid characterized by a carbon skeleton with four fused rings, generally arranged in a 6-6-6-5 fashion. Steroids vary by the functional groups attached to these rings and the oxidation state of the rings. Hundreds of distinct steroids are found in plants, animals, and fungi. All steroids are made in cells either from the sterol lanosterol (animals and fungi) or the sterol cycloartenol (plants). Both sterols are derived from the cyclization of the triterpene squalene.[1]