Variable age at the time of diabetes onset, which is actually determined by many factors [e.g., the type, position and penetration of causal mutation in HNF1A and HNF4A genes (29), presence of risk polygenic single nucleotide polymorphisms of type 2 diabetes (30) or exposure to hyperglycaemia in utero (13)] represents another problem in differential diagnosis. The gene discussed is HNF1A; the disease is type 2 diabetes mellitus.