 Cdt1 is a pivotal DNA replication initiation factor whose levels oscillate during cell cycle progression. In late G1, its levels rise to initiate DNA replication. At the beginning of S phase it is promptly degraded by the E3 ligase so that origins that are already fired once do not fire again in S phase and re-replication within a single cell cycle does not occur. Impaired degradation of Cdt1 in S phase by the E3 ligase is the major mechanism by which the cellular DNA undergoes re-replication. It was discovered that in E1A expressing cells Cdt1 levels remain high in S phase suggesting an inefficient degradation of Cdt1 which may contribute to extensive re-replication of cellular DNA that the investigators observed in late S phase. 