A group of polycyclic compounds closely related biochemically with terpenes. They contain cholesterol, a number of hormones, precursors of certain vitamins, bile acids, alcohols (sterols), and some natural medicines and poisons. Steroids have a common core, melted, reduced 17-kolatomen ring system, cyclopentanoperhydrophenanthrene. Most steroids also has two methyl groups and an aliphatic side chain attached to the core.