A R181P missense variant in CD40LG, the gene associated with X-linked hyper-IgM syndrome, was also predicted to be damaging but was inconsistent with the patient’s clinical and laboratory data including absent nibrin and a total serum IgG concentration in the normal range and positive IgG antibody responses to tetanus and H. influenzae b. Here, NBN is linked to X-linked hyper-IgM syndrome.