Estrone is one of the three naturally occurring estrogens, the others being estradiol and estriol. 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. Serum concentrations of estrone in premenopausal women fluctuate according to the menstral cycle and becomes the most predominant estrogen in postmenopausal women. The binding affinities of estrone to the estrogen receptors α and β (ERα and ERβ) are approximately 60% and 37% relative to estradiol.