ATR is a critical upstream regulator of checkpoint responses to incompletely replicated and damaged DNA. However, it had not been understood how the kinase activity of ATR is switched on during checkpoint responses. TopBP1 and its homologs are necessary for both DNA replication and checkpoint control. A recent report from this laboratory demonstrated that TopBP1 functions as an activator of ATR. It had been known that TopBP1 accumulates at sites of replicative stress and DNA damage. Thus, interaction of ATR with a critical protein at stalled replication forks and sites of DNA damage triggers its activation. This finding helps to explain how aberrant DNA structures in the genome induce ATR-dependent signaling processes.