Daidzein-d is intended for use as an internal standard for the quantification of daidzein by GC- or LC-MS. Daidzein is an isoflavone phytoestrogenic compound that has been found in soybeans and other legumes. It binds to estrogen receptor β (ERβ; K = 2.8 µM) but not ERα at concentrations up to 1 mM. It is estrogenic in vitro, increasing gene transcription mediated by the estrogen response element (ERE) in a reporter assay in an ERβ-dependent manner (EC = 2.8 µM for MCF-7 cells expressing ERβ). Daidzein is an inhibitor of carbonic anhydrase (CA) that is selective for carbonic CAVII and CAXII (Ks = 4.2 and 56 nM, respectively) over CAI, II, and IV (Ks = >10,000, >10,000, and 718.7 nM, respectively). It reduces tumor growth in a PC3 prostate cancer mouse orthotopic model when administered at a dose of 50 mg/kg per day and potentiates the effects of radiation therapy.