Noonan and LEOPARD syndromes are developmental disorders with overlapping features, including cardiac abnormalities, short stature and facial dysmorphia. Increased RAS signaling owing to PTPN11, SOS1 and KRAS mutations causes B60% of Noonan syndrome cases , and PTPN11 mutations cause 90% of LEOPARD syndrome cases 7 . Here, we report that 18 of 231 individuals with Noonan syndrome without known mutations (corresponding to 3% of all affected individuals) and two of six individuals with LEOPARD syndrome without PTPN11 mutations have missense mutations in RAF1, which encodes a serine-threonine kinase that activates MEK1 and MEK2. Most mutations altered a motif flanking Ser259, a residue critical for autoinhibition of RAF1 through 14-3-3 binding. Of 19 subjects with a RAF1 mutation in two hotspots, 18 (or 95%) showed hypertrophic cardiomyopathy (HCM), compared with the 18% prevalence of HCM among individuals with Noonan syndrome in general. Ectopically expressed RAF1 mutants from the two HCM hotspots had increased kinase activity and enhanced ERK activation, whereas non-HCM-associated mutants were kinase impaired. Our findings further implicate increased RAS signaling in pathological cardiomyocyte hypertrophy.Noonan syndrome is an autosomal dominant, genetically heterogeneous trait that can be caused by mutations in PTPN11, SOS1 or KRAS, which encode proteins participating in RAS-mitogen-activated protein kinase (MAPK) signal transduction 1,3-6 . SHP-2, encoded by PTPN11, and SOS1 have positive roles in RAS-MAPK signaling but possess complex autoinhibitory mechanisms that are altered by most mutations. KRAS, a small G protein, is activated by the exchange of bound GDP for GTP and possesses inefficient GTPase activity that is self inactivating; this inactivating activity can be significantly accelerated through interactions with GTPase-activating proteins (GAPs) such as neurofibromin. Noonan syndrome mutations in KRAS either reduce GTPase activity or are speculatively thought to enhance guanine nucleotide exchange 1,3 . Similarly, gain-of-function mutations have been found in HRAS in individuals with Costello syndrome and in KRAS in individuals with cardio-facio-cutaneous (CFC) syndrome, two disorders that resemble Noonan syndrome 8,9 . In addition, activating BRAF, MEK1 and MEK2 mutations have been observed in CFC 9,10 , whereas a distinct class of PTPN11 mutations engendering loss of catalytic activity underlies LEOPARD syndrome . Taken as a whole, disease pathogenesis for Noonan syndrome and related disorders entails altered RAS-MAPK signaling, predominantly through increased signal traffic.Missense mutations in PTPN11, the first gene associated with Noonan syndrome, are observed in approximately 50% of individuals with this disorder 12 . Pulmonary valve stenosis is more prevalent among individuals with Noonan syndrome with PTPN11 mutation, whereas HCM is quite rare. SOS1 and KRAS mutations, which are found in B10% of individuals with Noonan syndrome, are associated