Mammalian Chk1 is an essential kinase for embryonic development and plays an important role in the cellular response to DNA damage. However, it remains unclear whether inhibition of Chk1 induces apoptosis in somatic cells. The uncertainty has become a critical issue for rationale design of Chk1 mechanism-based anticancer drugs. Here we show that Chk1 small interfering RNA (siRNA) effectively eliminates Chk1 protein expression without altering the cell cycle profile or inducing apoptosis in various human cancer cell lines under normal conditions. In the presence of DNA-damaging agents, however, Chk1 siRNA alone is sufficient to abrogate the DNA damage-induced G(2) checkpoint and significantly enhance apoptosis. Cell cycle kinetic profiles show that abrogation of G(2) arrest is mediated through shortening of the checkpoint. We also demonstrate that Chk1 siRNA enhances DNA damage-induced apoptosis in p53-deficient cancer cell lines and augments the growth inhibition conferred by DNA-damaging agents. These findings imply that Chk1 inhibitors will have low cytotoxicity on their own and can enhance the efficacy of DNA-damaging drugs.