RNA interference technology is a powerful tool for silencing endogenous or exogenous genes in mammalian cells. Here our results showed that hdm2-siRNA silenced its target mRNA specifically and effectively in human breast cancer cells, reduced tumor cell proliferation and induced apoptotic cell death. Other molecular features modified by hdm2-siRNA included decreased Bcl-2, NF-kB, survivin, Ras and Raf levels, elevated p53, p21, BRCA1, Bax, and caspase levels as well as altered expression of other genes. hdm2-siRNA also caused cell cycle arrest at G1 phases with reduction in cyclin and Cdk proteins. In addition, hdm2-siRNA displayed in vivo antitumor activity and increased therapeutic effectiveness of mitomycin in MCF-7 xenografts. Thus, hdm2-siRNA may be a promising gene-specific drug for the treatment of human breast cancer and other tumors. Cancer Gene Therapy (2004) 11, 748–756. doi: 10. 1038/sj.cgt. 7700753