Steroids are characterized by which structural feature?

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Steroids are built on a four-ring fused backbone: three six-membered rings and one five-membered ring locked together in a rigid tetracyclic framework. This four-ring core, called the cyclopentanoperhydrophenanthrene nucleus, is the defining feature of the steroid structure and gives the molecules their characteristic hydrophobic, membrane-interacting properties. Different steroids differ by the groups attached to this core, but the four-ring skeleton is what they all share. In contrast, a long hydrocarbon chain is typical of many fats, a single benzene ring is just one aromatic ring, and a helical backbone is a feature more associated with proteins.