In addition, mutations in the receptors for B-cell activating factor (BAFF), BAFF receptor(BAFF-R) and transmembrane activator and calcium modulator and cyclophilin ligand interactor(TACI) lead to hypogammaglobulinemia (56–60) and TACI mutations are also related to sIgA deficiency (59, 60). The gene discussed is TNFRSF13B; the disease is hyperinsulinemic hypoglycemia, familial, 4.