ATR enforcement of the S/G2 checkpoint prevents premature S phase shutdown and genome instability

The ATR-enforced S/G2 checkpoint activates during DNA replication to restrain CDK1-dependent phosphorylation of FOXM1 and subsequent transactivation of the G2/M gene network until the end of S phase. However, the extent to which this checkpoint ensures the completion of DNA replication and whether it safeguards genomic integrity has remained unknown. Here, we induce S/G2 checkpoint failure throughout S phase in non-malignant human epithelial cells using multiple ATR pathway inhibitors. Consequently, the mitotic kinase complex cyclin B1-CDK1 prematurely shuts-down the DNA replication program, preventing the completion of genome duplication. In turn, this leads to the retention of inactive rep