Replication stress in cancer: origins, consequences and therapeutic opportunities.

Replication stress, defined as impaired DNA replication leading to genomic instability, is a central feature of tumour development and progression. In cancer cells, it arises from oncogene activation, transcription-replication conflicts, altered nucleotide metabolism and DNA secondary structures that hinder DNA synthesis. These pressures compromise accurate genome duplication and promote chromosomal instability and mutagenesis. Recent advances have clarified the molecular mechanisms that stabilize, remodel and restart stressed replication forks, and highlighted the essential role of the ataxia telangiectasia and Rad3-related protein (ATR)-checkpoint kinase 1 (CHK1) pathway in preventing excessive single-stranded DNA accumulation and catastrophic genome fragmentation. Although replication stress fuels tumour evolution and heterogeneity, it also creates dependencies on replication stress response pathways that can be therapeutically exploited. Inhibitors targeting ATR, CHK1 and WEE1 kinases, which override the G2 checkpoint, are in clinical development, whereas synthetic lethal strategies, including poly(ADP-ribose) polymerase (PARP) inhibition in BRCA1-deficient or BRCA2-deficient tumours and inhibition of the DNA helicase WRN in microsatellite-unstable cancers, illustrate how replication stress-associated vulnerabilities can be targeted. In this Review, we position replication stress as both a driver of cancer genome evolution and a tractable framework for precision oncology. We emphasize that replication stress in tumours is often chronic and sublethal, shaping genome evolution. We further highlight its integration with oncogenic and metabolic alterations and broader impact on tumour-microenvironment interactions, which create dynamic and context-dependent therapeutic vulnerabilities.