Decreased levels of IgM have been associatedwith episodes of recurrent infection, thymic hypoplasia, celiac disease,autoimmune disease, and certain adult malignancies and several primaryimmunodeficiencies (Wiskott-Aldrich Syndrome, ataxia-telangiectasia, CVID,Bruton agammaglobulinemia, SIgMID, combined IgG and IgM immunodeficiency, andtransient hypogammaglobulinemia of infancy) and congenital disorders (Bloomsyndrome and Russell-Silver syndrome).Isolated reduced serum IgM levels secondary to these other illnessesshould be excluded before making a diagnosis of primary SIgMID. The gene discussed is CD40LG; the disease is ataxia telangiectasia.