Chk1 is a serine/threonine-protein kinase involved in cell cycle arrest when DNA damage has occurred, and it plays an important role in DNA damage checkpoint, embryonic development and tumor suppression. Chk1 is activated in response to replication blocks and genotoxic stress, via phosphorylation of serines 317 and 345. Activated Chk1 can inactivate cdc25c through phosphorylation at Serine 216. Phosphorylation of cdc25c prevents activation of the CDC2-cyclin B complex and prevents entry into M-phase.