Poly(ADP-ribose) binding to Chk1 at stalled replication forks is required for S-phase checkpoint activation

DNA damage at stalled replication forks activates Chk1 kinase and poly(ADP-ribose) (PAR) polymerase 1. Min et al. find that retention of Chk1 to stalled replication forks depends on its direct interaction with PAR, and show that PAR chain length fine-tunes Chk1 and S-phase checkpoint activation.