(S)-Equol, or (S)-(–)-4',7-isoflavandiol, is the (S)-enantiomer of the naturally occurring isoflavandiol estrogen, equol. It is a non-steroidal, selective agonist of ERβ (Ki = 16 nM), with 13-fold selectivity for ERβ over ERα. Relative to (S)-equol, (R)-equol is less potent and, in contrast, binds to ERα (Ki = 50 nM) with 3.5-fold selectivity over ERβ. Under the code name AUS-131, (S)-equol is under development for the treatment of menopausal symptoms such as hot flashes and benign prostatic hyperplasia. Human intestinal bacteria metabolize the soy isoflavone daidzein into (S)-equol, and it has been determined that (S)-equol is exclusively produced.