Individuals carrying two ATM alleles with loss-of-function mutations are affected with Ataxia-Telangiectasia (A-T), which is an autosomal recessive syndrome characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, immunodeficiency, hypersensitivity to ionizing radiation, and cancer susceptibility (Gatti et al. 1991; Savitsky et al. 1995a; Ambrose and Gatti 2013). Here, ATM is linked to Immunodeficiency.