Claspin is a checkpoint protein that interacts with and regulates checkpoint kinase 1 (Chk1) in response to replication stress. The interaction between claspin and Chk1 has been demonstrated to be required for ATR-dependent Chk1 activation. The interaction involves the phosphorylation of claspin that may be accomplished by Chk1 kinase activity. Claspin has been found to localize to chromatin in both the presence and absence of damaged DNA and to associate with PCNA, as an essential component of the DNA replication machinery. This association with the replication machinery may indicate a role for claspin in linking replication to DNA damage signaling and Chk1 activation.