CD40LG and hyperinsulinemic hypoglycemia, familial, 4: In conclusion, diagnostic delay is a clinical problem for patients with <i>CD40LG</i> deficiency, because of low or normal IgM levels, showing that all the hypogammaglobulinemic patients, not only with high serum IgM levels, but also with normal to low IgM levels, have to be examined for <i>CD40LG</i> expression on activated T lymphocytes.