Whereas carriers of monoallelic ATM mutations (ATM+/− heterozygote) have slightly elevated cancer risks and increased radiosensitivity, the complete lack of ATM function results in the clinical syndrome AT (ATM−/− homozygote), characterized by progressive neuromotor dysfunction, immunodeficiency, genomic instability, predisposition to cancer, and profound hypersensitivity to ionizing radiation [18]. Here, ATM is linked to immunodeficiency disease.