Cell cycle checkpoints are protective mechanism responding to DNA damages originated from externalor internal factors. When cells are exposed to genotoxic stress or when nutrition crisis occurs, cell cycleprogression is usually stopped or slowed down by cell cycle checkpoints to allow for DNA repair or for handlingthe crisis. Besides, recent studies suggest that some cell cycle checkpoint proteins are also involved in regulatingphysiological DNA replication via controlling the rate of DNA replication. Cell cycle checkpoint proteins ATR,9-1-1 complex, Chk1, Cdc25A and CDK2 may participate in this process. This kind of regulation is supposed to bevery important for ensuring accurate DNA replication and maintaining genomic stability.