Genistein is an isoflavone found in soy products that interferes with estrogen's ability to interact with and encourage the growth of certain tumor cells.

Cancer cells need a steady supply of fresh blood in order to survive and thrive. Genistein interferes with blood supply to tumors, and may keep growth in check.

References

Messina, M., Barnes, S., and Setchell, K.D.R. 1997. Phytoestrogens and breast cancer. Lancet 350: 971- 972, Albertazzi, P., Pansini, F., Zhou JR, Gugger ET, Tanaka T, Guo Y, Blackburn GL, Clinton SK Soybean phytochemicals inhibit the growth of transplantable human prostate carcinoma and tumor angiogenesis in miceJ Nutr 1999 Sep;129(9):1628-35.