Selective immunoglobulin A deficiency (SIgAD) is defined as the decreased or absent level of serum immunoglobulin A (IgA) in the presence of normal serum levels of immunoglobulin G (IgG) and immunoglobulin M (IgM) in a patient older than 4 years of age, in whom other causes of hypogammaglobulinemia have been excluded [1]. The gene discussed is CD40LG; the disease is selective IgA deficiency disease.