One of the cellular alterations required for development of cancer is "genomic instability," a term that describes a collection of pathways all leading to increased frequency of gene mutation and altered patterns of gene expression. Among the mechanisms causing genomic instability are chromosome breakage, gene amplification, and changes in levels of gene expression. The latter are frequently associated with changes in gene methylation status. All of these mechanisms have been correlated with changes in replication timing, and it is possible that in some cases changes in replication timing may be the primary cause of these phenomena. So far it has been difficult to evaluate the role of changes in replication timing in cancer development, because so little is known about the mechanisms controlling replication timing. Fortunately previous studies in budding and fission yeasts (Saccharomyces cerevisiae and Schizosaccharomyces pombe), in our laboratory and others, provide entry points into understanding replication timing. In both yeasts we have identified replication origins containing internal cis-acting sequences that determine late replication timing. Here we propose simple strategies that will permit us to precisely localize those sequences and use them to pull out the corresponding trans-acting proteins. In addition, studies in other laboratories and ours have already identified several proteins likely to play roles in replication timing. We intend to investigate the potential roles of all of these proteins in replication timing by studying the effects on timing of mutations in the corresponding genes and by studying the binary interactions between these proteins. The results of the proposed experiments should permit us to develop detailed models of the mechanisms controlling replication timing in both budding and fission yeasts. The conserved aspects of these models are likely to prove applicable to other eukaryotic cells, including human cells. This information may permit the development of drugs that will enhance normal replication timing stability and thus inhibit or reverse the development of cancer.