Cdc25 is a protein phosphatase responsible for dephosphorylating and activating cdc2, a crucial step in regulating the entry of all eukaryotic cells into the M-phase of the cell cycle. Cdc25 is phosphorylated throughout interphase but not in mitosis. The primary site for phosphorylation of cdc25 is at Ser216. Phosphorylated cdc25 binds to members of the 14-3-3 family of proteins which prevents its activation of cdc2. Chk1, a protein kinase involved in the DNA damage checkpoint response in fission yeast, has been shown to phosphorylate cdc25 at Ser216 in vitro.
