They both have normal total lymphocyte counts, and serum IgM levels within the normal range, however serum IgG and IgA were undetectable in P1 wherease P2 had IgG2 subclass deficiency and low/normal IgA levels prior to commencing IVIg (Table 1). The gene discussed is CD40LG; the disease is hyperinsulinemic hypoglycemia, familial, 4.