CD4 and severe combined immunodeficiency: [42-45]; the "bubble boy paradox" (IL-2 deficient knockout mice and the rare IL-2 deficient human patients do not have SCID but instead severe defect of CD4+ TReg lymphocytes and autoimmunity) was resolved by the discovery that IL-2 receptor γ chain is shared (common γ chain) by the receptors of IL-4, IL-7, IL-9, IL-15, and IL-21; IL-7 and IL-15 are essential for the development of T lymphocytes and NK cells, respectively [46,47].