In the latest large studies conducted among Polish patients with a suspected diagnosis of MD, the highest number of heterozygous causative variants was confirmed in the GCK and HNF1A genes [9, 10], which is consistent with the results obtained in the similar studies in France [11], Russia [12], Lithuania [13], North America [14] and Canada [15]. The gene discussed is HNF1A; the disease is Menkes disease.