CDT1 (Chromatin Licensing and DNA replication Factor 1) is an essential component of the cell cycle machinery. It plays an important role in insuring that there is only one round of DNA replication per cell cycle. CDT1 is a known substrate of Cdk2 and Cdk4 (Liu, E. 2004). It is presented in G1 of the cell cycle and is degraded as cells enter S phase (Liu, E. 2004). In higher eukaryotes, CDT1 is known to interact with Geminin, a protein that is degraded during mitosis, that inhibits it. CDT1 is marked for degradation by two E3 ligases: SCF complex and the CUL4-DDB1 complex (Sansam, 2006).