Although most severe CHI are due to mutations in both genes ABCC8 and KCNJ11[1]–[3], other mutations have also been identified in the glucokinase gene (GCK) [4], the glutamate dehydrogenase gene [5], (GLUD1), the short-chain L-3-hydroxyacyl-CoA dehydrogenase gene (SCHAD) [6], the insulin receptor gene [7], the HNF4A gene in the setting of maturity-onset diabetes (MODY) [8], and the SLC16A1 gene which encodes monocarboxylate transporter 1 (MCT1) in the setting of exercise-induced hypoglycaemia [9]. The gene discussed is SLC16A1; the disease is congenital isolated hyperinsulinism.