This became clearer by examining patients with mutations in IL2RG, encoding γc, or JAK3, which associates with γc and delivers signals downstream of γc-containing cytokine receptors (73), that cause X-linked severe combined immunodeficiency (X-SCID) or one type of autosomal recessive (AR) SCID, respectively (73, 119). Here, JAK3 is linked to severe combined immunodeficiency.