It is vital for cells to effectively repair DNA damage and to coordinate DNA repair with cell cycle progression. Detection of DNA damage and transduction of this signal to downstream effectors is required for the maintenance of genomic integrity and suppression of cancer in humans. ATM (Ataxia Telangiectasia Mutated) is a kinase that is essential in coordinating the cellular response to DNA damage. ATM is recruited to DNA double strand breaks (DSBs) by the Mre11-Nbs1-Rad50 complex, while 53BP1 and MDC1 also facilitate ATM activation at DNA damage sites. ATM and ATR (ATM and Rad 3-related) phosphorylate and activate several downstream effectors for apoptosis, cell cycle arrest and DNA repair, including CHK1, CHK2, BRCA1/BRCA2, CtIP, p53, Mdm2, RPA, c-Abl and Rad51.