Observations: Mosaicism may produce different cutaneous patterns such as the lines of Blaschko, the checkerboard pattern, the phylloid pattern, and a patchy pattern without midline separation. A unique lateralization pattern is observed in the CHILD syndrome. Two major genetic categories are functional mosaics resulting from X inactivation and genomic mosaics caused by autosomal mutations. Functional mosaicism may be caused by either male-lethal or nonlethal X-linked mutations. Similarly, autosomal mutations resulting in genomic mosaicism may be either lethal or nonlethal. Many mosaics are caused by loss of heterozygosity, and uncommonly this mechanism may give rise to twin spots such as vascular twin nevi. Some cutaneous mosaic phenotypes virtually always occur sporadically, but exceptionally may show a familial aggregation. This paradox may be explained by paradominant inheritance. Heterozygous individuals are, as a rule, unaffected, but they express the birthmark when allelic loss occurs during embryogenesis.

Conclusions: The concept of cutaneous mosaicism is important for gene mapping because here we have the opportunity to study two populations of cells differing only with regard to the mutation causing mosaicism. Future research will probably show that a specific genetic anomaly, when present as a mosaic, always produces the same type of cutaneous pattern.