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Canine cutaneous histiocytoma (CCH) is the most common skin tumour of young dogs. CCH is assumed to be a tumour, but as these lesions often undergo complete regression, they have also been proposed to represent hamartomas or unregulated hyperplasia of polyclonal histiocytic cells. To demonstrate a monoclonal origin of CCH, and thereby a probable neoplastic nature, a clonality assay was established based on the heterozygosity of a polymorphic microsatellite within the canine androgen receptor located on the X chromosome. Analysis of 11 histiocytomas identified a non-random X chromosome inactivation of one androgen receptor allele. CCH may, therefore, be a clonal lesion and of neoplastic nature.