A few years after the discovery of mutations in the IL2RG gene encoding the common gamma-chain (γc) as the cause of X-linked severe combined immunodeficiency (X-SCID), an immunodeficiency characterized by the absence of T cells and NK cells in the presence of non-functional B cells [276], JAK3 mutations were identified as the cause of autosomal recessive T−B+NK–SCID [277, 278]. Here, JAK3 is linked to immunodeficiency disease.