Concept of twin spotting.

This article describes the formation of apparent twin spots presumed to be caused by a specific form of somatic recombination. Twin spots consist of two genetically different clones of neighboring cells in a background of normal cells. The phenomenon is well known in plants and animals and is used as a marker to evaluate the recombinogenic activity of chemicals. The equivalent of the twin spot phenomenon in humans has only been described recently. We now give a review on a number of paired skin disorders possibly caused by the mechanism of twin spotting. They include vascular twin nevi, phacomatosis pigmentovascularis, phacomatosis pigmentokeratotica, Proteus syndrome, and cutis tricolor. Clinicians will probably spot other nevoid skin lesions occurring in close proximity to each other, which might be explained by the twin spot phenomenon. Copyright 1999 Wiley-Liss, Inc.