Laboratory findings of these individuals partially overlapped with NIK deficiency or RelB deficiency, including B cell lymphopenia, reduced serum IgM and IgG (but normal levels of IgA), poor responses to pneumococcal polysaccharides vaccines, an accumulation of transitional B cells and corresponding paucity of total and class-switched memory B cells in peripheral blood. The gene discussed is RELB; the disease is hyperinsulinemic hypoglycemia, familial, 4.