Quantitation of serum immunoglobulin at 6 months by electrochemilumescent immunoassay confirmed that SMUG1 deficiency on its own does not increase the levels of IgM (Fig. 2B), nor does it affect the accumulation of switched isotypes (although as in the case of UNG deficiency, individual titers are quite scattered) (Fig. 2C). The gene discussed is SMUG1; the disease is hyperinsulinemic hypoglycemia, familial, 4.