A single heterozygous mutation was identified in 14 (67%) patients; 5 patients had maternally inherited ABCC8 mutations, 2 patients had de novo ABCC8 mutations (no mutations identified in parents), 1 patient had a paternally inherited ABCC8 mutation without focal CHI, 5 patients had paternally inherited KCNJ11 mutations without focal CHI and 1 patient had a maternally inherited KCNJ11 mutation. The gene discussed is KCNJ11; the disease is congenital isolated hyperinsulinism.