Maturity onset diabetes of the young (MODY) is a monogenic, autosomal dominant form of diabetes characterised by mutations in genes resulting in dysfunction of pancreatic β-cells and subsequent insulin production. We present a family with HNF1A-MODY due to a likely pathogenic mutation in HNF1A (c.59G>A, p.Gly20Glu), diagnosed a long time after the first diagnosis of diabetes. Currently 13 MODY subtypes caused by mutations in 13 genes, are known. We describe the four most prevalent forms in more detail, i.e. HNF4A-MODY, GCK-MODY, HNF1A-MODY and HNF1B-MODY, together responsible for probably 99% of MODY cases. The different forms of MODY vary in prevalence, severity of diabetes, occurrence and severity of diabetic complications and response to treatment. New tools, such as the MODY probability calculator, may be of assistance in finding those patients in whom further genetic testing for possible MODY is warranted. However, as our described family shows, a doctor’s clinical eye and taking the time for a detailed family history may be equal to, or even better than, the best prediction rule.