Maturity-onset diabetes of the young (MODY) is the most common type of monogenic diabetes mellitus, estimated to account for approximately 1-4% of patients with diabetes. The predicted prevalence is, therefore, 20,000 patients in The Netherlands. Unfortunately less than 5% of these patients are confirmed by molecular genetic analysis. MODY is a clinically heterogeneous group of disorders caused by β-cell dysfunction, which is caused by mutations in multiple genes. MODY is characterized by an early onset of diabetes (often before the age of 30 years) and autosomal dominant inheritance. Patients do not usually require insulin at diagnosis. To emphasize the importance of genetic analysis we describe a 7-year-old boy and his siblings with MODY type 2. Molecular genetic testing is essential for individual patient care, as treatment options differ between the various forms of MODY; it also provides an opportunity to screen relatives.