Required for checkpoint mediated cell cycle arrest in response to inhibition of DNA replication or to DNA damage induced by UV irradiation. Adapter protein which binds to the checkpoint kinase chek1 and facilitates the phosphorylation and activation of this kinase by the ATR-TREX1/ATRIP kinase complex. Also associates with chromatin during S-phase, and this requires the pre-replication complex (pre-RC), cdc45 and cdk2. This may indicate a role for this protein as a sensor which monitors the integrity of DNA replication forks. Interacts with both chek1 and plk1/plx1 when phosphorylated. The CHEK1-binding domain (CKBD) contains 2 potential CHEK1-binding motifs (CKB motifs). Phosphorylation sites within CKB motif 1 and CKB motif 2 are required for interaction with CHEK1. Phosphorylated. Phosphorylation at Ser-864, Ser-895 and Thr-906 occurs during checkpoint activation and requires the ATR kinase. Phosphorylation at Ser-864 within CKB motif 1 and Ser-895 within CKB motif 2 promotes interaction with the catalytic domain of chek1 and subsequent activation of the kinase. Phosphorylation at Thr-906 creates a binding site for the Polo-box domain of the plk1/plx1 kinase, which subsequently phosphorylates Ser-934. Phosphorylation of Ser-934 is required for chek1 inactivation during checkpoint adaptation, the process whereby a prolonged cell cycle arrest is alleviated without prior removal of the checkpoint-inducing DNA lesion. Checkpoint adaptation may promote cellular proliferation when non-lethal DNA damage cannot be repaired in a timely manner. Belongs to the claspin family.