TP53 and cancer: Loss of G1 checkpoint control, through mutations in the TP53 or RB tumor suppressor genes, activation of oncogenes such as Ras or MYC, or imbalance in G1/S cyclins, cyclin-dependent kinases (CDKs) and their inhibitors, is a common feature of cancer cells (6), making these cells dependent on the G2 checkpoint for survival after DNA-damaging treatments.