The Cdc6 protein is an essential component of pre-replication complexes (preRCs), which assemble at origins of DNA replication during the G1 phase of the cell cycle. Cdc6 participates in checkpoint controls that ensure DNA replication is completed before mitosis is initiated. It has been concluded that expression of Cdc6 is regulated in response to mitogenic signals though transcriptional control mechanisms involving E2F proteins, and that Cdc6 is required for initiation of DNA replication in mammalian cells. In vitro, Cdc6 is an excellent substrate for Cdk2, phosphorylated at three sites (Serine 54, Serine 74 and Serine 106).