Unilaterally clustered facial melanocytic nevi in either of dizygotic twins.

Abstract

A pair of dizygotic twin sisters was presented at the age of 11 years because they had developed during the previous years a peculiar distribution of acquired melanocytic nevi on their face, being present in the one twin on the right side, and in the other one on the left side only. Moreover, several small melanocytic nevi involving the trunk and the limbs on either side were noted in both twins. We infer that the unilaterally clustered nevi of these twins may be explained as a superimposed segmental manifestation of a polygenic trait being characterized by bilaterally distributed, scattered melanocytic nevi. We infer that the more pronounced unilateral involvement may have originated from an early mutational event giving rise either to loss of heterozygosity at one of the predisposing loci, or to heterozygosity for a predisposing allele at an additional locus.