Cutaneous mosaicism is a complex field of Dermatology which encompasses most birthmarks, and many rare syndromes. Mosaic abnormality of the skin are the coexistence of cells with at least two genotypes, at least one of which is pathological, by the time of birth, in an individual derived from a single zygote, and which leads to a disease phenotype. Very few treatment options are available for most mosaic abnormalities of the skin but within the current decade high sensitivity genetic techniques have led to an explosion of knowledge about genotype and phenotype in the literature. This has led to fundamental changes in the understanding of the conditions in this field, and to an ongoing paradigm shift in the approach to management of mosaic skin disorders.

As part of the ERN-Skin project the field of mosaic cutaneous disorders has been highlighted as an area where guidelines for patient management are scarce, and new publications on genetic aetiology are appearing at a remarkable rate, making it difficult for practitioners to keep up to date with the latest knowledge. The thematic group on Cutaneous Mosaic disorders thus aims to help the practitioners provide the best care to their patients by reviewing the latest trials of targeted therapies in mosaic disorders, giving them practical tips on correct sample collection and genetic investigation for different mosaic abnormalities of the skin, providing guidelines and also reviewing the knowledge of the confirmed mosaic disorders.