Chk2 (also known as cell cycle checkpoint kinase 2) is a serine/threonine protein kinase containing an FHA domain. There are two reported isoforms of Chk2 (designated A and B) with molecular weights of approximately 60 kD. Chk2 can be modified by phosphorylation. This nuclear protein controls cell cycle checkpoint and inhibits Cdc25C by phosphorylation. Chk2 prevents entry into mitosis and is upregulated by autophosphorylation in response to DNA damage or other agents that cause replication block. Chk2 has been shown to interact with BRCA1, p53, and Cdc25C. The CHK2909 monoclonal antibody has been shown to be useful for Western blotting of the human Chk2 protein.