GCK-MODY cases in the top HbA<sub>1c</sub> quintile had a 3-to-6-fold risk of exceeding the diabetes diagnostic HbA<sub>1c</sub> threshold (≥48 mmol/mol) in clinically selected and clinically unselected cohort respectively.<h4>Conclusions/interpretation</h4>Our findings suggest that polygenic background and GCK variants interact to modify the glycaemic expression of GCK-MODY, influencing clinical diagnosis despite high penetrance. This evidence concerns the gene GCK and diabetes mellitus.