DNA replication has to be tightly regulated to ensure genome integrity such that DNA replication takes place only once per cell cycle. The Cdc6 sequential phosphorylation by GSK-3 and Cdk1 creates a binding site for Cdc4 ubiquitin ligase to promote Cdc6 degradation.
Al-Zain, A., Schroeder, L., Sheglov, A. & Ikui, A. E. (2015). Cdc6 degradation requires phosphodegron created by GSK-3 and Cdk1 for SCFCdc4 recognition in Saccharomyces cerevisiae. Molecular Biology of the Cell, 26(14), 2609-2619. doi:10.1091/mbc.E14-07-1213.