SIgAD is defined by a serum IgA level below 7mg/dL in individuals four years and older in whom serum IgG and IgM are normal, and for whom other causes of hypogammaglobulinaemia have been excluded [2]; some authors also allow for the presence of IgG subclass deficiency in SIgAD [3,4]. The gene discussed is CD40LG; the disease is selective IgA deficiency disease.