ATM and Immunodeficiency: Homozygous germline inactivation of ATM causes the Ataxia-telangiectasia (A-T) syndrome, characterized by oculocutaneous telangiectasia (vascular dilation), cerebellar ataxia, immunodeficiency, greatly increased risk for cancer, especially lymphoid malignancies, and extreme sensitivity to ionizing radiation (IR) [87–89].