Three (4%) patients carried pathogenic variants in genes previously linked with other cancer syndromes (ATR, BRIP1 and MSH6) and another three cases carried monoallelic pathogenic variants in recessive cancer genes (xeroderma pigmentosum and Fanconi anaemia), indicating that reduced penetrance of phenotype in these individuals may contribute to the development of both UM and CM. Here, MSH6 is linked to cutaneous mastocytosis.