The S phase checkpoint: when the crowd meets at the fork.

Accumulation of unrepaired DNA lesions is the biggest threat to genomic stability. DNA damage checkpoints create windows of time that allow the cell to repair assaults on DNA in each phase of the cell cycle. When DNA lesions arise in S phase, however, the checkpoint machinery must work to coordinate DNA replication and repair processes. In fact some upstream components of the DNA damage checkpoint play parallel roles in maintaining the continuity of DNA replication and signaling to downstream components.