HAE is most frequently caused by mutations in the SERPING1 gene coding for C1 inhibitor (C1INH), resulting in quantitative C1INH deficiency (HAE Type I) or C1INH dysfunction (HAE Type II), collectively known as HAE due to C1INH deficiency (HAE-C1INH).2 Here, SERPING1 is linked to hyperinsulinemic hypoglycemia, familial, 4.