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 HNF4A; the disease is diabetes mellitus.