Soy isoflavones are phytoestrogens found in soybeans. Phytoestrogens are plant-derived nonsteroidal compounds that possess estrogen-like biological activity. Soy isoflavones have both weak estrogenic and weak anti-estrogenic effects. They have been found to bind to estrogen receptors-alpha (ER-alpha) and beta (ER-beta). They appear to bind better to ER-beta than to ER-alpha. Soy isoflavones comprise three main isoflavones and their glycosylated forms. The three main isoflavones are the aglycones genistein, daidzein and glycitein.