It has been proposed that the atypical mole syndrome has either an autosomaldominant mode of inheritance with incomplete penetrance or a polygenic modeof inheritance.2 The specific gene or genesthat predispose to the development of dysplastic nevi and the precipitantsfor the formation of individual melanocytic nevi have not been identified.However, sun exposure has been implicated in the development of melanocyticnevi.24 Furthermore, it has been postulatedthat some atypical melanocytic nevi may develop through loss of heterozygosity(LOH).25,26 Loss of heterozygosityis the loss of a normal wild-type allele, leading to the expression of a mutantor recessive allele.27 One mechanism for acquiredloss of an allele may be through mutations induced by UV exposure.28 The specific etiology of agminated nevi is not known.Local environmental factors, such as UV radiation, may play a role in thedevelopment of clustered lesions. However, it is also possible that this agminatedpattern may represent LOH, occurring during embryogenesis, as has been postulatedto account for the segmental distribution of lesions such as neurofibromas,porokeratosis, and Becker nevi.27 In our patient,the agminated dysplastic nevi occurred in the presence of the atypical molesyndrome. Other examples of segmental clustering of skin lesions superimposedon a generalized less severe form of the condition may shed light on the pathogenesisin our case. Autosomal dominant skin disorders, such as neurofibromatosis,sometimes occur in a segmental pattern superimposed on a less severe but diffuseform of the same disorder.27 This phenomenonmay be explained by an early postzygotic mutational event giving rise to LOHat the locus responsible for the trait. A person can also have a segmentalmanifestation of a polygenic skin disorder, such as segmental severe psoriasis,superimposed on symmetric involvement of ordinary psoriasis.27This too can be explained by LOH occurring in a somatic cell during earlyembryogenesis, resulting in either homozygosity or hemizygosity for one ofthe genes predisposing to psoriasis. Future studies of the distribution oflesions in cases of agminated nevi such as ours coupled with genetic analysisof microdissected tissues may shed light on the timing and nature of theseevents.