Eukaryotic cells duplicate their chromosomes. Errors in the DNA replication control could lead to chromosome instability. Our long-term goal is to understand the molecular mechanism of DNA replication control in eukaryotes. Cdc6p is a key DNA replication component from yeast to humans. Our preliminary results showed that yeast GSK-3 kinase (Mck1p) targets DNA replication factor Cdc6p for degradation, which in turn inhibits DNA re-replication. It is known that N-terminal region of Cdc6 is phosphorylated by Cyclin/CDK complex for its degradation after the origin is licensed. We will study the novel Cdc6p degradation mechanism controlled by Mck1p kinase to inhibit DNA re- replication. In this proposal, three aims will test my hypothesis if a sequential phosphorylation of Cdc6p by Cdk1 and GSK3 controls the timing of Cdc6p degradation. Aim1 will test if Cdc6p is directly phosphorylated by Mck1 kinase, and Mck1-depedent Cdc6 degradation is mediated through SCFCDC4 complex. Aim2 will test the possibility that Mck1p targets Cdc6p in a distinct mechanism from CDK. Aim3 will test if the phosphorylation of Cdc6 by CDK is a pre-requisite for phosphorylation by Mck1p. Cdc6p and GSK-3 kinase are conserved from yeast to humans. The results obtained from this study can be applied to understand the molecular mechanism of Cdc6 degradation by GSK-3 kinase in higher eukaryotes. The accurate DNA replication control is a critical step to avoid chromosome instability, and it is important to understand since the chromosome instability is a hallmark of tumorigenesis.