Estrone is one of the three naturally occurring estrogens, the others being estradiol and estriol.{9611} Estrone is synthesized from androstenedione by the aromatase enzyme system in the ovaries and placenta, and is also synthesized from estradiol by 17-hydroxy steroid dehydrogenase in the liver.{9611,8343} Serum concentrations of estrone in premenopausal women fluctuate according to the menstrual cycle and becomes the most predominant estrogen in postmenopausal women.{9611} The binding affinities of estrone to the estrogen receptors α and β (ERα and ERβ) are approximately 60% and 37% relative to estradiol.{9611}