ABSTRACT
A “mosaic” is an organism composed of two or more cells that are genetically different, derived from a genetically homogenous zygote. The distinction between mosaicism of lethal from nonlethal postzygotic mutations is important. Other factors, such as the timing of mutation during embryo development and the cells affected by the mutation, are also key factors in determining the depth of mosaicism and its clinical expression. In this chapter we discuss genetic and epigenetic causes underlying mosaicism and classic patterns of cutaneous mosaicisms. We describe briefly the most important conditions that can produce a mosaic hypopigmentation in the skin, including hypomelanosis following Blaschko lines, segmental hypomelanosis arranged in a checkerboard pattern, epidermal nevus, incontinentia pigmenti, Pallister-Killian syndrome, phylloid hypomelanosis, Conradi-Hünermann-Happle syndrome, and Goltz syndrome.
