Naturally occurring compounds and synthetic analogues, based on the cyclopenta[a]phenanthrene carbon skeleton, partially or completely hydrogenated; there are usually methyl groups at \(\ce{C-10}\) and \(\ce{C-13}\), and often an alkyl group at \(\ce{C-17}\). By extension, one or more bond scissions, ring expansions, and/or ring contractions of the skeleton may have occurred.