In Eukaryotic cells, cell cycle checkpoint regulation ensures the fidelity of cell division. This kind of control verifies whether the processes at each phase of the cell cycle have been accurately completed before progression into the next phase. Mitogen-dependent progression through the first gap phase (G1) of the mammalian cell-division cycle is precisely regulated so that normal cell division is synchronous with cell growth. Also, the initiation of DNA synthesis (S phase) has to be timed precisely to avoid inappropriate DNA amplification which can result in genomic instability. The G1/S cell cycle checkpoint controls the passage of eukaryotic cells from the G1 into the S phase...