Claspin is a replication fork-associated protein important for Chk1 activation. Claspin loads onto the fork during replication origin firing and travels with the fork during DNA synthesis. Upon fork uncoupling and ATR-ATRIP binding to persistent ssDNA, the activated ATR kinase phosphorylates claspin at two primary sites. Modification increases the affinity of claspin for Chk1. Studies of human or Xenopus claspin indicate that phosphorylation of both sites is essential for significant claspin-Chk1 association. Following claspin modification by ATR, Chk1 can be transiently recruited to the stalled replication fork for subsequent phosphorylation and activation by ATR. Activation of Chk1 allows modification of additional downstream targets, thus amplifying the checkpoint signal. While much of the mechanistic information concerning claspin action has been obtained using Xenopus laevis egg extracts and Xenopus claspin, factors with similar activity have been found in various eukaryotic species including S. cerevisiae (MRC1), S. pombe (mrc1), and humans.

Activated ATR phosphorylates human claspin on two sites, threonine 916 and serine 945.