We found that downregulation of Chk using small interfering RNA (siRNA-Chk) significantly reduced proliferation in breast cancer cells50,51 and tumors.52 Combining siRNA-Chk with 5-FU treatment resulted in additional reduction of cell viability and proliferation in breast cancer cells.51 Recently, a novel potent and selective small-molecule Chk-α inhibitor, V-11–0711, which acts by inhibiting the catalytic activity of Chk-α,53 was evaluated in HeLa cells. This evidence concerns the gene CHKA and breast carcinoma.