This BCERF Brief will describe the existing evidence for a link between soy phytoestrogen exposure and breast cancer risk. Estrogen is one of the major reproductive hormones in women. It is also a hormone that can cause breast cancer in both humans and animals.Phytoestrogens are similar chemicals produced by various plants, such as soybeans, flaxseeds and sesame seeds. These three plants produce large amounts of phytoestrogens, with some of the highest levels being found in soy beans. Many processed foods, including some protein bars, breakfast cereals, cookies, multigrain breads, and doughnuts also have high phytoestrogen levels because soy is added to these foods.