X-linked severe combined immunodeficiency (SCID-X1) is caused by deficiency of the common cytokine receptor gamma chain, a component of multiple cytokine receptors (IL2, IL4, IL7, IL9, IL15 and IL21) responsible for many aspects of lymphoid development and function10,11. The gene discussed is IL4; the disease is T-B+ severe combined immunodeficiency due to gamma chain deficiency.