BACKGROUND: Human carcinomas not only consist of neoplastic epithelial cells but also of tumor stroma, which may play an important role in tumor-progression. Whereas the tumor surrounding stroma is generally believed to represent a reactive component induced by tumor cell derived factors, a contribution of neoplastic cells to stroma formation via epithelium-mesenchyme transition during tumor invasion has become a novel concept in recent years. MATERIALS, METHODS AND RESULTS: We here show, by laser-assisted microdissection, that frequent genetic alterations in non-hereditary invasive human colon and breast cancers (loss of heterozygosity and TP53 mutations) occur not only in the neoplastic epithelial cells, but also in the adjacent fibroblastic stroma and that both components can share clonal features. CONCLUSION: Tumor cell-mesenchyme transitions are among the possible explanations for these findings and could actually occur during tumor invasion in vivo.