Noonan syndrome (NS) is a relatively frequent autosomal dominant disorder characterised by facial dysmorphic features, heart defects, developmental delay, and short stature. This syndrome is mostly caused by gain-of-function mutations in the PTPN11 gene, encoding tyrosine phosphatase. The best-defined consequence of NS-causing mutants is an enhancement of Ras/MAPK activation that is responsible for the different NS features. Mutations in several genes encoding other components of the Ras/Mitogen Activated Protein Kinase (MAPK) pathway, resulting in hyperactivation, are also found in syndromes close to NS.