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Noonan syndrome (NS) is one of the most frequent dysmorphic syndromes in children with a frequency of 1/1000-1/2500 of newborns. Noonan syndrome is a multi-organ disease with a broad spectrum of clinical symptoms. The most characteristic features of NS are: craniofacial dysmorphy, short stature, cardiovascular defects, bone and skeletal defects and delayed puberty (cryptorchidism in males). Noonan syndrome has a genetic background and is inherited in autosomal dominant manner. The recent studies have shown that it is due to the presence of mutation in one of the genes encoding proteins of RAS/MAPK signalling pathway responsible for cell proliferation and differentiation. Till now, NS causing mutations were identified in PTPN11, SOS1, RAF1, KRAS, BRAF, SHOC2 and NRAS genes, and this may partially explain the broad phenotypic spectrum observed in patients. Noonan syndrome is one of the RAS-opathies, therefore the molecular analysis of RAS/ MAPK genes might be a very useful tool in clinical differentiation of the disease.