X-linked SCID, accounting for 40%–50% of SCID cases reported world-wide, is caused by mutations in the IL2RG gene leading to defective expression of the common gamma chain (γc), a subunit shared by a host of cytokine receptors including interleukin (IL)-2, 4, 7, 9, 15 and 21 receptor complexes which play a vital role in lymphocyte development and function. The gene discussed is IL2RG; the disease is severe combined immunodeficiency.