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16α-Hydroxyestrone (16α-OH-E1), or hydroxyestrone, also known as estra-1,3,5(10)-trien-3,16α-diol-17-one, is an endogenous steroidal estrogen and a major metabolite of estrone, as well as an intermediate in the biosynthesis of estriol. It is a potent estrogen similarly to estrone, and it has been suggested that the ratio of 16α-hydroxyestrone to 2-hydroxyestrone, the latter being much less estrogenic in comparison and even antiestrogenic in the presence of more potent estrogens like estradiol, may be involved in the pathophysiology of breast cancer. Conversely, 16α-hydroxyestrone may help to protect against osteoporosis. In contrast to estradiol, the binding of 16α-hydroxyestrone to the estrogen receptor is, uniquely, covalent and irreversible, and genotoxicity and aberrant hyperproliferations may result. A diacetate ester of 16α-hydroxyestrone, hydroxyestrone diacetate, has been marketed and is used medically as an estrogen in Europe.