<p>This entry represents 17beta-hydroxysteroid dehydrogenases (17B-HSDs), a group of enzymes which catalyse the last step in the biosynthesis of all androgens and estrogens -the reversible NAD(P)-linked transfer of a hydride to and from the 17-position of steroid molecules [<cite idref="PUB00028053"/>]. A total of six isozymes have been identified which vary in substrate specificity, tissue specificity and preferred direction of the reaction.</p><p>The most intensively studied enzyme in this entry is human estrogenic 17beta-hydroxysteroid dehydrogenase (<db_xref db="SWISSPROT" dbkey="P14061"/>) which is responsible for the last step in the synthesis of all estrogens. As active estrogens stimulate the proliferation of breast cancer cells, this enzyme is a potential target for drugs to treat breast cancer [<cite idref="PUB00024639"/>]. It is a membrane-associated homodimer which posseses the Tyr-X-X-X-Lys motif typical of short-chain dehydrogenases and forms a typical Rossman fold [<cite idref="PUB00023623"/>].</p>