It is well known that autosomal dominant skin disorders may sometimes become manifest in a mosaic form, involving the body in a linear, patchy, or otherwise circumscribed arrangement. Such cases can be explained by an early postzygotic mutation. The segmental lesions usually show the same degree of severity as that found in the corresponding nonmosaic trait. Occasionally, however, the intensity of involvement observed in the circumscribed area is far more pronounced. This phenomenon can be explained by delineating a rule of dichotomous segmental manifestations reflecting different states of zygosity. Heterozygosity for the mutation results in severity corresponding to that in the nonsegmental phenotype; loss of heterozygosity for the same allele causes markedly more severe involvement.