ATM has important roles in regulating DNA DSB repair, cell cycle checkpoint activation and DNA damage-induced apoptotic pathways. Patients with the progressive neurodegenerative disorder ataxia telangiectasia, with biallelic mutation of ATM, show both a cellular and clinical hypersensitivity to ionizing radiation. They also show a variable immunodeficiency; immune abnormalities may include low serum IgA, IgE and selective IgG subtypes, elevated IgM and lymphopenia affecting T cells in particular and decreased immune repertoire diversity, resulting from impaired V(D)J and class switch recombination. There is an increased tendency for sino-pulmonary infections due to the immunodeficiency and pulmonary disease is a major cause of death. The second major cause of death is an increased predisposition to lymphoid tumours. Given the role of physiologically-induced DSBs in promoting somatic recombination in the adaptive immune system, and that a failure to repair DSBs generated during the latter process is known to facilitate lymphoid tumourigenesis, some aspects of the clinical phenotype of A-T correspond with the underlying cellular DNA repair defect. But patients with A-T also show an increased risk of carcinoma including tumours in the gastrointestinal tract, endocrine tumours and female breast cancer in adulthood indicating a wider involvement of ATM in cellular processes.