The ataxia telangiectasia mutated (ATM) gene is important in the cellular response to DNA double stranded breaks. ATM genetic abnormalities include deletions and mutations which may occur alone or together. ATM mutations have been reported to occur in approximately 35% of chronic lymphocytic leukemia; biallelic ATM inactivation (deletions and mutations) are associated with a poor outcome in some setttings compared to monoallelic ATM deletion or mutation in CLL. The ATM mutations occur throughout the gene and are putative loss of function variants (missense, nonsense and frameshift mutations). ATM mutations have also been reported in other types of lymphoma including mantle cell lymphoma (50%) and T cell prolymphocytic leukemia (46%).