Introduction Genistein is an isoflavone phytoestrogen that induces cell differentiation in tissue culture Constantinou and Huberman (1995). An inhibitor of tyrosine kinase, histidine kinase, and topoisomerase Akiyama et al (1987), Huang et al (1992), Barnes and Petersen (1995), genistein is thought to be an anticancer, antiproliferative, cardioprotective and/or chemopreventive agent. It is possible, however, that the enzyme inhibitory effects may only occur at non-physiological concentrations Belenky et al (2003). At physiological concentrations, genistein activates the nuclear estrogen receptors ER-alpha and ER-beta Kuiper et al (1997), Kuiper et al (1998), Casanova et al (1999) and affects TGF-beta signaling pathways Kim et al (1998). While genistein, or its more soluble glucoside derivative genistin, are taken as a dietary supplement, they have not been thoroughly tested in clinical trials. Common dietary supplements often include other isoflavones, such as daidzein and glycitein or their corresponding glucoside conjugates.