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 t wo genetically different clones of neighboring cells in a background of nor mal cells, The phenomenon is well known in plants and animals and is used a s a marker to evaluate the recombinogenic activity of chemicals. The equiva lent 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, phacom atosis pigmentovascularis, phacomatosis pigmentokeratotica, Proteus syndrom e, and cutis tricolor. Clinicians will probably spot other nevoid skin lesi ons occurring in close proximity to each other, which might be explained by the twin spot phenomenon, (C) 1999 Wiley-Liss, Inc.