Hypogammaglobulinemia was identified based on simultaneously decreased IgG, IgA and IgM serum levels in 67% of cases (including case 9 with 665 mg/dL IgG, close to the lower limit of normality), while in the remaining 3 cases a defect of only two Ig isotypes (IgG/IgA or IgG/IgM) was present. Here, CD40LG is linked to agammaglobulinemia.