General blood test results were unremarkable, but raised levels of serum IgM (5.63 g/l; normal range, 0.56-2.61) associated with low serum concentration of IgG and IgA (6.06 g/l; normal age-matched range, 6.33-10.16; and 0.33 g/l; normal range, 0.41-3.15, respectively) suggested the diagnosis of HIGM, supported by a marked reduction of in vivo switched memory B cells. The gene discussed is CD40LG; the disease is hyper-IgM syndrome type 1.