Cdc6 is a regulator of DNA replication initiation in both yeasts and human cells, but its mechanism of action differs between the two systems. Genetic studies in budding yeast (S. cerevisiae) and fission yeast (S. pombe) indicate that the normal function of Cdc6 protein is required to restrict DNA replication to once per cell cycle. Specifically, Cdc6 may function as an ATPase switch linked to Mcm2-7 association with the Cdt1:Cdc6:ORC:origin complex. In S. cerevisiae, Cdc6 protein is expressed late in the M phase of the cell cycle and, in cells with a prolonged G1 phase, late in G1. This protein has a short half-life, and is destroyed by ubiquitin-mediated proteolysis, mediated by the SCF complex. Human Cdc6 protein levels are reduced early in G1 but otherwise are constant throughout the cell cycle. Some reports have suggested that after cells enter S phase, Cdc6 is phosphorylated, excluded from the nucleus and subject to ubiquitination and degradation. Replenishing Cdc6 protein levels during G1 appears to be regulated by E2F transcription factors.