Collapse of G2 to G1 after DNA damage is initiated by p21-mediated CDK2 inhibition and rendered irreversible by the destruction of G2/M-cyclins A and B. This behaviour is altered in cells with cancer-associated mutations in the p53-phosphatase WIP1 (PPM1D), which increase the threshold for DNA-damage signalling, enabling DNA-damaged G2 cells to enter mitosis with elevated levels of MDM2, thereby suppressing mitotic-timer-dependent G1 cell-cycle arrest. This evidence concerns the gene CDK2 and cancer.