Cutaneous mosaicism of lethal mutations.

The concept of autosomal lethal genes surviving only in a mosaic state was proposed by Happle to explain the genetic basis of several syndromes characterized by (almost always) sporadic occurrence, distribution of lesions in a scattered or asymmetrical pattern, variable extent of involvement, lack of diffuse involvement of entire organs, and equal sex ratio. The mosaic may either arise from a gametic half-chromatid mutation or from an early postzygotic mutation. The purpose of this article is to review current knowledge of the genetics and cutaneous manifestations of some of the birth defects to which the lethal gene concept is thought to apply: the Schimmelpenning (Feuerstein-Mims) syndrome, Proteus syndrome, encephalocraniocutaneous lipomatosis, Sturge-Weber and Klippel-Trenaunay syndrome, cutis marmorata teleangiectatica congenita (van Lohuizen syndrome), and neurocutaneous melanosis.