Whole exome sequencing revealed that Case 1 is carrying a novel homozygous mutation in <i>CYP11B2</i>, c.1391_1393dup p.(Leu464dup) and Case 2 has a homozygous pathogenic variant in <i>CYP11B2</i>, c.922T>C p.(Ser308Pro), confirming the diagnosis of CMO 1 deficiency in both cases. The gene discussed is CYP11B2; the disease is hyperinsulinemic hypoglycemia, familial, 4.