The large predominance of males among IEIs cases is certainly due to the presence of the severe X-linked IEIs, such as SCID due to IL2RG mutations, IPEX, WAS, hyper-IgM due to CD40L deficiency, nuclear factor κB essential modulator (NEMO), and most chronic granulomatous disease and agammaglobulinemia cases which tend to have a severe clinical phenotype with symptoms manifesting in the first year of life [140], and in agreement to the Jeffrey Modell Foundation global report and a systematic review that included 104,614 IEIs patients registered in 80 countries [134, 141]. The gene discussed is CD40LG; the disease is severe combined immunodeficiency.