Blaschko lines and other patterns of cutaneous mosaicism

The lines of Blaschko represent a classic pattern of cutaneous mosaicism that can be observed in a wide variety of congenital and acquired skin disorders. This contribution reviews the clinicopathologic spectrum of skin lesions that follow Blaschko lines. Four other patterns of mosaicism are also discussed: blocklike, phylloid, large patches without midline separation, and lateralization. We emphasize the differential diagnoses, clues to correct categorization, and associated findings of inflammatory, hypopigmented, and hyperpigmented lesions with a mosaic distribution. Clinical examples are used to illustrate genetic concepts such as functional X-chromosome mosaicism, type 1 and 2 segmental manifestations of autosomal dominant skin diseases, paradominant inheritance, and twin spotting.