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Steroids are a class of naturally occurring organic substances that are of great importance in biology, medicine, and chemistry. The steroid group includes all the sex hormones, adrenal cortical hormones, bile acids, and sterols of vertebrates, the molting hormones of insects, and many other physiologically active substances of animals and plants. Among the synthetic steroids of therapeutic importance are a large number of anti-inflammatory agents, anabolic (growth stimulating) agents, and oral contraceptives. All steroids are related to a characteristic molecular structure composed of 17 carbon atoms arranged in four rings conventionally denoted by the letters A, B, C, and D. These four carbon rings are then additionally bonded to 28 hydrogen atoms. This parent structure, named gonane and often referred to as the steroid nucleus, may be modified in a practically unlimited number of ways by removal, replacement, or addition of a f