Purity >98%. Equol is a nonsteroidal estrogen produced from the metabolism of the isoflavonoid phytoestrogen daidzein by human intestinal microflora. The estrogen receptor (ER) binding activity of the naturally occurring (S)-enantiomer demonstrates greater affinity toward ERBeta while the (R)-enantiomer demonstrates greater affinity towards ERAlpha. Synthesized as a racemic mixture, (±)-equol exhibits EC50 values of 200 and 74 nM for human ERAlpha and ERBeta, respectively and induces breast cancer cell proliferation in vitro at concentrations as low as 100 nM.