Equol is a non-steroidal estrogen produced from the metabolism of the isoflavonoid phytoestrogen daidzen by human intestinal microflora. The estrogen receptor (ER) agonist activity of the naturally occurring (S)-enantiomer (EC50 = 85 and 65 nM for human ERα and ERβ, respectively) is similar to that of genistein but exceeds that of daidzein. (S)-Equol preferentially binds ERβ (Ki = 0.73 nM) and demonstrates approximately 9-fold lower affinity for ERα (Ki = 6.41 nM). (S)-equol is also a potent antagonist of dihydrotestosterone, which has important implications for prostate cancer and other androgen-mediated pathologies.