Several genetic diseases can have the clinical features of either a generalized condition or a disorder with an alternating pattern of affected and unaffected body segments. With skin disorders, in which the segmental expression of symptoms is most easily observed, the pattern of affected and healthy skin often follows the so-called lines of Blaschko. Such mosaic patterns have long been assumed to reflect mutations occurring after fertilization. These postzygotic mutations result in genetic mosaicism -- a condition in which people have a mixture of genetically distinct cell populations (Figure 1). The article by Paller and colleagues in this issue of . . .