 This is partly because the CDC6 gene is only transcribed during G1 phase.On the onset of the S phase, CDC6 gets phosphorylated by the Cdc28-Clb5-Clb6 complex (Cdk2) and consequently becoming inactivated.This has been shown by introducing mutations in CDC6 at the consensus sites for Cdk2 phosphorylation (near the N-terminus) which inhibit degradation.