Mosaicism happens when mutation occurs post-zygotically (i.e. post-conception) or when it affects the X chromosome in a female individual due to lyonization. Pattern observed depends on the stage of embryogenesis when the mutation occurs (earlier – more extensive) and the cells involved. If mutation happened in a melanoblast, dyspigmentation along lines of Blaschko or block-like or phylloid pattern or congenital melanocytic nevi are seen.

Mutation affecting the keratinocytes also follows the lines of Blashko. On the other hand, mesodermal cells (i.e. fibroblasts) and neurons usually follow segmental or dermatomal pattern.

Type 1 mosaicism is defined when there is an autosomal dominant mutation (“gain of heterozygosity”) whereas type 2 mosaicism is seen in autosomal dominant mutation with a second hit or “loss of heterozygosity.” Rarely a second hit mutation can restore normal phenotype which is called revertant mosaicism. Selected XLD disorders are presented here, but many other genetic diseases that are not found on this page may present in a mosaic form.

These other diseases are discussed in detail in other sections (e.g. ichthyoses).