CHEK2 (Checkpoint kinase 2) is a tumor suppressor gene that encodes the protein CHK2, a serine-threonine kinase. CHK2 is mainly involved in DNA repair, cell cycle arrest or apoptosis in response to DNA damage. When DNA undergoes a double-strand break, CHK2 is activated. Specifically, DNA damage-activated phosphatidylinositol kinase family protein (PIKK) ATM phosphorylates site Thr68 and activates CHK2. Once activated, CHK2 phosphorylates downstream targets including CDC25 phosphatases, responsible for dephosphorylating and activating the cyclin-dependent kinases (CDKs). Thus, CHK2’ s inhibition of the CDC25 phosphatases prevents entry of the cell into mitosis.