The β-catenin and APC genes are key components of the Wnt signaling pathway. Mutation of these genes results in increased levels of the β-catenin protein, which is associated with enhanced cellular proliferation and the development of both colon polyps and colon cancer. Recently, a technique known as RNA interference has been successfully adapted to mammalian cells so that it is now possible to specifically decrease the expression of cellular genes after transfection of annealed small interfering 21-mer RNAs. In the current study, we used small interfering RNA (siRNA) directed against β-catenin to determine the effects of decreasing the high constitutive levels of this protein in colon cancer cell lines with mutations in either β-catenin or APC. Our studies demonstrate that siRNA directed against β-catenin markedly decreased β-catenin-dependent gene expression and inhibited cellular proliferation as reflected in the reduced growth of these colon cancer cells both in soft agar and in nude mice. These results indicate that siRNA can target specific factors whose expression is altered in malignancy and may have the potential as a therapeutic modality to treat human cancer.