DNA damage, caused by UV light, ionizing radiation, environmental toxins, and metabolic chemicals, can also lead to genomic instability and cancer. The DNA damage response to double strand DNA breaks (DSB) is mediated in part by histone modifications. At a DSB, MRE11-RAD50-NBS1 (MRN) protein complex recruits ataxia telangiectasia mutated (ATM) kinase which phosphorylates Serine 129 of Histone 2A.