Urticaria Pigmentosa in Identical Twins

Urticaria pigmentosa (UP), or cutaneous mastocytosis, is the most common mast cell disease of childhood. It produces no visceral lesions or systemic symptoms and usually improves by adolescence.1 During the last 100 years, the literature has reported cases of various forms of mastocytosis in 40 families in which more than one member was affected.2Among these cases were 11 sets of twins and one set of triplets. Seven pairs of identical twins and the triplets were concordant for the disorder, while two identical twin pairs and the two fraternal sets were discordant. We report an eighth set of identical twins who both have UP, and we discuss the genetics of this condition. Report of Cases Twins 1 and 2 were black, male infants who were seen at the age of 4 months. Their parents were not related, and there was no family history of skin disease. The delivering