Identical sequence variants in both the epithelial and mesenchymal components (as well as in the corresponding normal tissue) were found in 80 % of the benign mixed tumours and in 89 % of the carcinomas arising from benign mixed tumours suggesting a shared clonal origin. The distinctive sequence alterations identified in the epithelial and mesenchymal components of 15.8 % of all 19 tumours examined, suggests the possibility that a minority of mammary tumours are polyclonal in origin or that early clonal divergence occurs. Increased mutation within the mtDNA D-loop fragment of mixed tumour components was not observed.abs>