Keratinocyte epidermal nevi (KEN) are benign congenital skin lesions that result from postzygotic mutations in mosaicism. Activating oncogenic mutations in FGFR3 and PIK3CA have been reported in approximately 40% of KEN and are absent from normal skin. However, the cause of the remaining cases is not known. Hafner et al. report that postzygotic mutations in RAS genes, the most common oncogenes involved in human cancer, are associated with a large fraction of KEN and the HRAS p.G13R substitution is a new hotspot mutation involved in this lesion. Therefore, KEN represent examples of “mosaic RASopathies”. (By Dr. Francisco X Real, http://jmg.bmj.com/content/49/4/249.abstract)