Tumors arise from genetic changes that transform normal growth-limited cells into highly invasive cells that are unresponsive to controls that restrain cell division. The genetic evolution that drives cancer is largely determined by the rate of DNA damage balanced against the fidelity of DNA replication and repair. The G2-M DNA damage checkpoint, which restrains the onset of mitosis in response to DNA damage, is an important part of the mechanism that maintains genome integrity. This checkpoint provides time to repair DNA before cell division. The checkpoint control system also regulates DNA repair, and in mammalian cells it may control apoptosis. The long-term objective of the research described in this proposal is to achieve a fundamental understanding of how eukaryotic organisms respond to DNA damage. These studies will be carried out with the fission yeast Schizosaccharomyces pombe. Cell cycle and checkpoint controls are remarkably conserved between fission yeast and humans. The project has four specific aims that address major unsolved questions concerning the G2-M DNA damage checkpoint. One aim is to uncover how the checkpoint effector kinase Chk1 is regulated. The second aim is to better understand the regulation and function of the checkpoint mediator protein Crb2, a BRCT-domain protein that appears to be the functional analog of human tumor suppressor BRCA1 protein. The third aim is to uncover how Chk1 is regulated during the cell cycle. The fourth aim is reveal how Chk1 regulates that mitotic inhibitor kinase Mik1 and to discover novel targets of Chk1 and Crb2. Insights gained from these studies will establish a framework for investigating genome integrity mechanisms in humans that will provide long-term benefits to human health.