Epidermal naevi are localized malformations of the epidermis consisting of verrucoid scaly papules and plaques following Blaschko's lines. Genetic mosaicism has been proposed to underlie the development of linear epidermal naevi. Rarely, epidermal naevi show acantholytic histology similar to Darier's disease, a dominantly inherited skin condition characterized by widespread warty papules. As patients with acantholytic dyskeratotic naevi often give a history of worsening after sun exposure and the lesions are typical of Darier's disease, numerous authors have proposed that these patients have segmental Darier's disease. The postulated relationship has not been proven, however. Recently, we identified ATP2A2, which encodes the sarco/endoplasmic reticulum Ca(2+) ATPase isoform 2 as the defective gene in Darier's disease. In this report, we investigated the involvement of ATP2A2 in acantholytic dyskeratotic naevi following Blaschko's lines in two patients. We identified a nonsense mutation (Y894X) in the first patient and a nonconservative glycine to arginine mutation at codon 769 (G769R) in the other patient. These mutations were present in affected skin, and were not detected in unaffected skin or in leukocytes. We conclude that acantholytic dyskeratotic naevi can arise from a somatic mutation in ATP2A2. These individuals are mosaics for the mutation, but the risk of transmission of generalized Darier's disease will depend on whether the germline is affected. Our findings provide further evidence that Blaschko's lines do reflect genetic mosaicism and that the term acantholytic dyskeratotic naevus might be replaced in the future by segmental Darier's disease induced by postzygotic mosaicism. J Invest Dermatol 115:1144-1147 2000