X-linked severe combined immunodeficiency (X-SCID) is caused by mutations of <i>IL2RG</i>, the gene encoding the interleukin common gamma chain (IL-2Rγ or γc) of cytokine receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Here, IL21 is linked to T-B+ severe combined immunodeficiency due to gamma chain deficiency.