© 2012 Springer-Verlag Berlin Heidelberg. All rights are reserved. Legius syndrome is an autosomal dominant disorder caused by heterozygous inactivating mutations in the SPRED1 gene. It is characterised by multiple café-au-lait macules (CALM) with or without axillary or inguinal freckling and mild macrocephaly. Learning disabilities, developmental delay and behaviour problems have been reported in Legius syndrome although less frequently than in NF1. The typical NF1-associated tumours are not evident in Legius syndrome. SPRED1 is a negative regulator of the RAS-MAPK pathway downstream of RAS, and pathogenic mutations in SPRED1 result in hyperactivation of the RAS-MAPK pathway. CALM in individuals with Legius syndrome are the result of bi-allelic SPRED1 inactivation in melanocytes. Legius syndrome belongs to the group of RASopathies which are developmental disorders resulting from germline mutations in genes coding for essential components of the RAS-MAPK pathway.