CHK1 (Serine/threonine-protein kinase CHK1) is required for checkpoint-medated cell cycle arrest and activation of DNA repair in response to the presence of DNA damage or unreplicated DNA. CHK1 phosphorylates Cdc25, an important phosphatase in cell cycle control,Porcinearticularly for entry into mitosis. Cdc25, when phosphorylated on serine 216 by Chk1 becomes bound by an adaptor protein in the cytoplasm. Therefore it is inhibited from removing the inhibiting phosphate from MPF (mitotic/maturation promoting factor) added by Wee1. Consequently, a cell is prevented from entering mitosis. Chk1 can also phosphorylate p53 at Ser20 in vitro.