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—bonded to 28 hydrogen atoms. This parent structure (1), named gonane (also known as the steroid nucleus), may be modified in a practically unlimited number of ways by removal, replacement, or addition of a few atoms at a time; hundreds of steroids have been isolated from plants and animals, and thousands more have been prepared by chemical treatment of natural steroids or by synthesis from simpler compounds. The steroid nucleus is a three-dimensional structure, ...(100 of 7003 words)