Legius syndrome and NF1 are part of a comprehensive syndrome called RASopathy (Fig. 6B), which is caused by germline loss-of-function and gain-of-function mutations in genes encoding protein components of the Ras/MAPK pathway.78) These mutations result in excessive activation of the Ras-ERK pathway during the embryonic developmental stage. This evidence concerns the gene NF1 and Legius syndrome.