Depending on the genetic etiology, the various genetic subtypes differ in terms of age of onset, pattern of hyperglycemia, response to treatment, and extra-pancreatic manifestations. Most cases of MODY are due to mutations in three genes, hepatic nuclear factor 1 alpha (HNF1A), hepatic nuclear factor 4 alpha (HNF4A), and glucokinase (GCK), with a detection rate that varies among different study populations. The gene discussed is GCK; the disease is MODY.