Superimposed segmental manifestation of both rare and common cutaneous disorders: a new paradigm.

In autosomal dominant skin disorders, a superimposed mosaic involvement arranged in a linear or otherwise segmental pattern is sometimes noted. Molecular proof of such type 2 segmental manifestation has so far been provided in Hailey-Hailey disease and Cowden syndrome. A similar superimposed segmental involvement can be found in numerous common disorders with a polygenic background, such a psoriasis, lichen planus, or vitiligo. In polygenic diseases, however, we can never recognize with certainty a type 1 segmental manifestation, which is why we should use more neutral terms in the form of isolated versus superimposed segmental involvement. In the near future, the new paradigm of superimposed segmental manifestation may hopefully help elucidate the molecular basis of both monogenic and polygenic skin disorders.