Images removed due to copyright considerations. See the following: Figure 1 and Table 1 in Reed SI. 2003. Ratchets and clocks: The cell cycle nover ubiquitylation and protein turnover. Nat Rev. Mol. Cell Biol. 4: 855-864. Figure 1 in Bartek J, Lukas J. Mammalian G1-and S-phase checkpoints in response to DNA damage. Curr Opin Cell