Although exposure of rodents to soy isoflavones during the perinatal period appears to reduce mammary cancer formation, exposure in utero or during adulthood may increase tumor growth. This model is comparable with human breast cancer because overexpression of the neu oncogene occurs in 20-40% of human breast cancers.
Once tumors formed, however, the isoflavones did not reduce the number or size of tumors such that at 34 wk of age there were no differences in tumor burden among the treatment groups. Hence, in the MMTV-neu mouse, soy isoflavones delayed mammary tumorigenesis.