Of patients receiving IHC testing, 30.3% (422) had an abnormal result, of which 79.9% had MLH1 deficiency (with or without PMS2, MSH2, MSH6 deficiencies), 8.8% had isolated MSH6 deficiency, 7.1% had combined MSH2 and MSH6 deficiencies, 3.3% had isolated PMS2 deficiency, 0.5% had isolated MSH2 deficiency and 0.5% had combined MSH6 and PMS2 deficiency. The gene discussed is MSH2; the disease is hyperinsulinemic hypoglycemia, familial, 4.