“…Such cases have been reported, for example, in neurofibromatosis type 1 [Boltshauser et al, 19891, nevoid basal cell carcinoma syndrome [Shelley et al, 1969;Camisa et al, 19851 and epidermolytic hyperkeratosis of Brocq [Nazzaro et al, 1990;Paller et al, 19941. However, sometimes the degree of involvement of the confined area is far more pronounced and, notably, the segmental lesions are superimposed on a milder, diffuse manifestation of the same phenotype. Such dichotomous forms of expression have been described in cases showing a linear arrangement of neurofibromatosis [Archer et al, 19881, cutaneous leiomyomatosis [Rudner et al, 1964;Mezzadra, 1965;Berendes et al, 19711, epidermolytic hyperkeratosis of Brocq [Hadlich and Linse, 19891, or porokeratosis, a disorder characterized by circular skin lesions showing central atrophy and a tiny keratotic ridge [Moreland and Wyre, 1981;Commens and Shumack, 1987;Happle, 19911. To explain this phenomenon that has so far not attracted much attention, the following rule may be considered.…”