It now is apparent that a family of closely related genes encode for 3beta- hydroxysteroid dehydrogenase (3betaHSD). Studies on the regulation of these genes are in their infancy, but the regulation appears multifactorial. The various 3betaHSD genes are expressed principally in a tissue-specific manner likely involving separate mechanisms of regulation. To date, two human 3betaHSD genes and their products have been characterized; type I is expressed in placenta, sebaceous glands, and several other nonendocrine tissues, whereas the type II isoform is the principal 3betaHSD of adrenal cortex and gonads.