Reactive oxygen species (ROS) are ubiquitous in the human body and play a pivotal role in many chronic diseases, including cancer. The human body has developed a strong antioxidant defense system. Unfortunately, this system cannot be adequately maintained with ageing. The supply of exogenous antioxidants to help the body fight these diseases is a logical approach. Polyphenols, particularly flavonoids, have been found to be strong antioxidants. In this study, the prenylated isoflavone, pomiferin, from Osage orange, was examined. I found that pomiferin had a much stronger antioxidant activity than soy isoflavones, genistein and daidzein, in three different antioxidant assays including [beta]-CLAMS, FRAP and PCL. By using both estrogen receptor positive (MCF-7) and negative (MDA-MB-435) cancer cell lines, pomiferin showed much lower IC50 values than soy isoflavones in both cell lines (5.2 ± 0.90 vs 5.4 ± 0.72 M) after 24 hr treatment. The selectivity of pomiferin between cancer cells and MCF-10A (spontaneously immortalized human breast epithelial cell line) was high. Microarray techniques were used to find the gene expression changes when all three cells were treated with 5.0 M pomiferin. At p < 0.05, 515, 691 and 59 genes were significantly regulated for MCF-7, MDA-MB-435 and MCF-10A, respectively; and at p < 0.01 cut off, 94, 105 and 1 genes, respectively. Many of these genes are associated with antioxidant enzymes, cell cycle regulation and apoptosis. To confirm the results from microarray, RT-qPCR was used to verify some of the gene expression changes. The expression of 20 out of 21 genes was confirmed. In addition, the xenograft models of MCF-7 and MDA-MB-435 cells were established to find the 'in vivo' anticancer activity of pomiferin. The tumour size was significantly reduced in MCF-7 with 0.2% and MDA-MB-435 with 0.5% of pomiferin in their diet (p < 0.05). Bioavailability of pomiferin was measured and the correlation with tumour sizes was evaluated. Plasma pomiferin level can partially explain the tumour size, but many other factors likely play a role. Further studies are needed to determine the specific mechanism(s) by which pomiferin alters specific gene expression and the differential effects in tumour versus normal cells.