Study summary: The current study was designed to investigate the ability of various soy products containing genistin (the glycoside form of genistein) to affect the growth of MCF-7 breast cancer cells transplanted into ovariectomized mice. Soybean-based products consumed in Asian countries are minimally processed whereas in the U.S. many soy foods and soy ingredients are highly processed. The soy products investigated included soy flour, two crude extracts of soy (soy molasses and Novasoy®), a mixture of isoflavones, and genistin in pure form. Each of the soy flour products was added to the mouse diet in a manner designed to provide equivalent amounts of genistein equivalents. Tumors in the negative control animal group regressed throughout the study while the tumors in the soy flour-fed animals remained basically the same size (neither grew nor regressed). However, in mice consuming soy molasses, Novasoy®, mixed isoflavones or genistin alone, tumor growth was found to be stimulated compared to animals consuming a control diet devoid of soy. These dietary treatments also resulted in increased cellular proliferation. In further testing, changes in mRNA expression of gene targets (estrogen responsiveness, cell cycle progression, apoptosis and aromatase activity) in tumors induced by the different diets were evaluated. The relative expression of pS2, progesterone receptor and cyclin D1 was higher in animals consuming the Novasoy®, mixed isoflavones and genistin. Bcl2 mRNA expression was low in most of the dietary treatment groups compared to positive (estradiol implant) controls. Aromatase expression was not altered in any of the treatment groups. The authors conclude that the degree of soy flour processing impacts the estrogenicity of products containing a constant amount of genistein. The findings suggest that for postmenopausal women with estrogen-dependent breast cancer, consumption of foods containing soy flour is more advisable than consuming isoflavones in more purified forms.