Any of a group of compounds characterized by possession of the perhydrocyclopenteneophenanthrene skeleton, which is made up of a series of four carbon rings (see diagram). Steroids may be distinguished from certain terpenoids with a similar skeleton by their possession of methyl groups at carbons 10 and 13. All natural steroids also have an oxygen group at carbon 3. The majority of steroids are alcohols (see sterol
). Other types of steroid include certain of the pseudoalkaloids and the aglycone portions of the saponins and cardiac glycosides.