Segmental dysplastic nevi are nevi that are restricted to one area of the body3. This condition is much rarer than the occurrence of dysplastic nevi, but nonetheless, may involve the same genetic mutation. The pattern of distribution in SDN is thought to result from mosaicism, i.e. the part of the body that expresses the dysplastic nevi has a mutation in a gene, however, the rest of the body does not contain this same mutation. Other mosaic disorders that have been studied in greater detail than SDN have been shown to be caused by a single gene mutation4. It appears that SDN is no different and that it too is caused by a mutation in a single gene, however, this gene is not yet known.