In this study, we report a 4-month-old boy with T−B+NK− severe combined immunodeficiency (SCID) due to a novel mutation in exon 2 of IL2RG, the gene encoding the interleukin (IL) common gamma chain (γc) of the cytokine receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Here, IL21 is linked to severe combined immunodeficiency.