Variants in GCK and, surprisingly, INS were the most common causes for monogenic diabetes in our study, followed by variants in HNF1A, HNF4A and HNF1B. However, we are likely to underestimate the prevalence of GCK-MODY, as individuals with stable and mild hyperglycaemia might remain undiagnosed or they might be misclassified as having type 2 diabetes [35] or just not be registered in the FPDR. Here, HNF1B is linked to type 2 diabetes mellitus.