Pigmentary Mosaicism: An Overview

Mosaicism refers to an individual who developed from a single fertilised egg but has two or more populations of cells with a different genotype as a result of a postzygotic mutation. Pigmentary mosaicism is reflected by a patterned hypo-, hyperpigmentation, or both combined in cutis tricolour. Pigmentary mosaicism can be associated with extracutaneous features (mainly neurological, musculoskeletal or ophthalmological). Three main mechanisms are involved in the development of pigmentary mosaicism: mosaicism for a chromosomal abnormality, mosaicism for an intragenic pathogenic variant and epigenetic mosaicism (X-linked due to X-chromosome inactivation). Recently, different new disease entities have been described with a specific genotype, most of them with characteristic extra-cutaneous features.