Clinically, Omenn syndrome is not a "leaky" SCID and has a poor prognosis; it is an extremely serious T+ or T++ SCID with pathogenic child's (not maternal) autologous oligoclonal hyper-autoreactive CD4+ TH2 lymphocytes, produced because of non-null mutations and then expanded by lack of central and peripheral immunological tolerance (respectively: thymic defect of thymocyte-dependent epithelial and dendritic cells and of AIRE, Autoimmune Regulator Element, expression; and defect of CD4+TReg lymphocytes) [23,33-35]. Here, CD4 is linked to Omenn syndrome.