Background: The autosomal-dominant Noonan syndrome (MIM 163950) is characterized by short stature, heart defects, characteristic facial dysmorphic features and other major and minor anomalies. Its incidence has been estimated to be 1 in 1,000 to 2,500 live births. Familial cases are frequent. Methods and Results: Recently, molecular data have suggested that deregulation of signaling through the Ras-mitogen-activated protein kinase (Ras-MAPK) pathway was the main molecular basis of Noonan syndrome. The frequently detected upstream defects of this pathway are gain-of-function mutations of PTPN11, which are associated with a mild form of growth hormone (GH) resistance and insulin-like growth factor I (IGF-I) deficiency, presumably due to interference with the Janus kinase 2 and signal transducer and activator of transcription 5b (JAK2-STAT) signaling of the GH receptor. Present data suggest reduced GH responsiveness in these cases. Conclusions: Downstream defects of the Ras-MAPK pathway (like K-ras mutations) do not affect the JAK2-STAT pathway, and therefore response to GH therapy is likely to be better in these cases.