A steroid is a terpenoid lipid characterized by its sterane or steroid nucleus: 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 from the sterol cycloartenol (plants). Both sterols are derived from the cyclization of the triterpene squalene.
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