UNLABELLED: Medical history and clinical examination: We report on a 52-year-old lean female patient with diabetes mellitus, treated with various glucose-lowering medications including insulin for 6 years. Both her mother and great-grandmother were also diagnosed with diabetes. At admission, she presents with good glycemic control and no signs of diabetes-related comorbidities. LABORATORY TESTS: HbA1c is 6 %. On the basis of positive family history for diabetes, normal body weight and negative antibodies against islet-cell antigens, we perform blood molecular genetic analyses with the suspicion of a genetic form of diabetes. Genetic tests reveal a monogenic form of diabetes (MODY, Maturity Onset Diabetes of the Young) due to an heterozygous mutation in exon 6 of the glucokinase (GCK) gene. The same mutation is subsequently detected in the patient's mother. DIAGNOSIS, TREATMENT AND COURSE: We diagnose MODY2 diabetes due to a glucokinase gene mutation. Repaglinide therapy is withdrawn due to lack of solid literature data showing beneficial cardiometabolic outcomes in patients with diabetes. Based on the MODY diagnosis, we recommend sulfonylurea treatment, which is however rejected by the patient due to a history of hypoglycemic episodes under the lowest dose. Thus, sitagliptin-treatment is continued under regular visits in our outpatient clinic. CONCLUSIONS: The joint presence of a positive family history, age under 25 years at diagnosis of diabetes, normal bodyweight, negative diabetes-specific autoantibodies and mild hyperglycaemia should raise the suspicion of a monogenic form of diabetes such as MODY and result in molecular-genetic testing.