A checkpoint operating in the G 2 phase of the cell cycle prevents entry into mitosis in the
presence of DNA damage. UCN-01, a protein kinase inhibitor currently undergoing clinical
trials for cancer treatment, abrogates G 2 checkpoint function and sensitizes p53-defective
cancer cells to DNA-damaging agents. In most species, the G 2 checkpoint prevents the
Cdc25 phosphatase from removing inhibitory phosphate groups from the mitosis-promoting
kinase Cdc2. This is accomplished by maintaining Cdc25 in a phosphorylated form that …