TI - The ATM-Chk2-Cdc25A checkpoint pathway guards against radioresistant DNA synthesis . AB - When exposed to ionizing radiation ( IR ) , eukaryotic cells activate checkpoint pathways to delay the progression of the cell cycle . Defects in the IR -induced S-phase checkpoint cause 'radioresistant DNA synthesis' , a phenomenon that has been identified in cancer-prone patients suffering from ataxia-telangiectasia , a disease caused by mutations in the ATM gene . The Cdc25A phosphatase activates the cyclin-dependent kinase 2 ( Cdk2 ) needed for DNA synthesis , but becomes degraded in response to DNA damage or stalled replication . Here we report a functional link between ATM , the checkpoint signalling kinase Chk2/Cds1 ( Chk2 ) and Cdc25A , and implicate this mechanism in controlling the S-phase checkpoint . We show that IR -induced destruction of Cdc25A requires both ATM and the Chk2 -mediated phosphorylation of Cdc25A on serine 123 . An IR -induced loss of Cdc25A protein prevents dephosphorylation of Cdk2 and leads to a transient blockade of DNA replication . We also show that tumour-associated Chk2 alleles cannot bind or phosphorylate Cdc25A , and that cells expressing these Chk2 alleles , elevated Cdc25A or a Cdk2 mutant unable to undergo inhibitory phosphorylation ( Cdk2AF ) fail to inhibit DNA synthesis when irradiated . These results support Chk2 as a candidate tumour suppressor , and identify the ATM-Chk2-Cdc25A-Cdk2 pathway as a genomic integrity checkpoint that prevents radioresistant DNA synthesis .