The functional activity of C1-INH was assessed in 23 HAE patients with quantitative C1-INH deficiency, of whom 12 had levels below 50% (median: 28.9%; range: 3.4–48%), and 11 patients had normal values (median: 70%; range: 51–120%). The gene discussed is SERPING1; the disease is hyperinsulinemic hypoglycemia, familial, 4.