Urticaria pigmentosa occurred in only one member of a pair of uniovular twins observed at 22 years of age. The twin without skin lesions had neither clinical or roentgenologic evidence of systemic mastocytosis, nor elevation of 24-hour urinary excretion of free-histamine and mucopolysaccharides. Previous articles described six pairs of uniovular twins concordant for cutaneous mastocytosis and one pair of uniovular twins in addition to two pairs of fraternal twins discordant for this condition. Our patient with urticaria pigmentosa differed from the affected uniovular twins by later onset and persistence of lesions.
Selmanowitz VJ, Orentreich N, Tiangco CC, Demis DJ. Uniovular Twins Discordant for Cutaneous Mastocytosis. Arch Dermatol. 1970;102(1):34-41. doi:10.1001/archderm.1970.04000070036005