Equol is a metabolite produced from the soy phytoestrogen daidzein by the action of gut microflora. Equol has higher affinity for both ERs (estrogen receptors including ERalpha and ERbeta). Equol exists as the enantiomers R-equol and S-equol. S-equol has a high binding affinity, preferential for ERbeta with a Ki of 16 nM whereas R-equol binds more weakly and with a preference for ERalpha K with a Ki of 50 nM. Equol is superior to all other isoflavones in its antioxidant activity. Equol has antioestrogenic properties. Equol is a 100-fold more potent than daidzein in stimulating an oestrogenic response. Equol is also more effective than daidzein in competing with 3 H-oestradiol for binding to the ER. Equol stimulates the growth of MCF-7 cells in a concentration-dependent manner. Although equol exhibits oestrogenic activity, exposure of MCF-7 cells to equol simultaneously with oestradiol is effective in reducing pS2 mRNA expression. Equol results in the downregulation of ER mRNA expression.