A mosaic variant in MAP2K1 is associated with giant naevus-spilus type congenital melanocytic naevus and melanoma development.

Giant congenital melanocytic naevi (CMN) arise primarily due to postzygotic mutations in NRAS, and are of clinical importance due to their increased risk of malignant transformation.1 A subtype of large CMN, termed naevus spilus (NS)-type CMN, is identified by the presence of a patch which may have café-au-lait pigmentation and superimposed macular or papular lesions, which are demonstrably melanocytic naevi on histology.