Genistein-d4 is intended for use as an internal standard for the quantification of genistein by GC- or LC-MS. Genistein is an isoflavonoid phytoestrogen that has been found in soybeans (G. max/S. hispida) and has kinase inhibitory, anticancer, pro-cancer, hepatoprotective, and antiviral properties.1 It inhibits the tyrosine kinases EGFR, pp50v-Src, and pp110gag-fes (IC50 = 6, 7-8, and 6.5 μg/ml, respectively) and decreases EGF-induced serine, threonine, and tyrosine phosphorylation of EGFR in A431 cells when used at a concentration of 20 μg/ml.2 Genistein inhibits proliferation and induces apoptosis in a variety of cancer cells, including Bel 7402 hepatocellular carcinoma cells when used at a concentration of 10 μg/ml.1,3 It reduces tumor invasion and angiogenesis in a Bel 7402 mouse subrenal capsule xenograft model when administered at a dose of 50 mg/kg per day.3 However, when administered at the same dose on postnatal days 1-5, genistein increases the incidence of uterine adenocarcinoma in a mouse model of cancer induced by the estrogen receptor agonist diethylstilbestrol .4 It reduces lipid accumulation and inflammation in the liver of ovariectomized (OVX) and non-OVX female rats in a model of high-fat high-fructose diet-induced nonalcoholic hepatosteatosis (NASH) when administered at a dose of 16 mg/kg per day.5 Genistein (10 μM) also inhibits HIV-1 DNA synthesis in resting CD4+ T cells.6