Dietary genistein, a natural flavone compound found in soy, has been proposed to be responsible for the low rate of breast cancer in Asian women. The cellular mechanisms of genistein's chemopreventive effects in vio have been largely unexplored. In our previous studies, we found that genistein exerted pronounced antiproliferative effects on both estrogen receptor-positive and -negative human breast carcinoma cells through G2-M arrest, induction of p21WAF1/CIP1 expression, and apoptosis. Because chemopreventive effects need not be limited to antiproliferation, we decided to examine whether genistein exerted other suppressive effects on breast carcinoma progression. Genistein inhibited invasion in vitro of MCF-7 and MDA-MB-231 cells. This inhibition was characterized by down-regulation of MMP (matrix metalloproteinase)-9 and up-regulation of tissue inhibitor of metalloproteinase-1, the former of which was transcriptionally regulated at activation protein-1 sites in the MMP-9 promoter. Genistein's in vitro effects on MMP-9 and tissue inhibitor of metalloproteinase-1 were also demonstrated in in vivo studies in nude mouse xenografts of MDA-MB-231 and MCF-7 cells. In these xenograft studies, genistein inhibited tumor growth, stimulated apoptosis, and upregulated p21WAF1/CIP1 expression. In the MDA-MB-231 xenograft, genistein also inhibited angiogenesis by decreasing vessel density and decreasing the levels of vascular endothelial growth factor and transforming growth factor-beta1. These in vitro and in vivo studies demonstrate that genistein exerts multiple suppressive effects on breast carcinoma cells, suggesting that its mechanism of chemoprevention is pleiotropic.