Coexpression of the human Met receptor and its ligand, hepatocyte growth factor/scatter factor (HGF/SF), in NIH 3T3 fibroblasts causes the cells to become tumorigenic in nude mice. The resultant tumors display lumen-like morphology, contain carcinoma-like focal areas with intercellular junctions resembling desmosomes, and coexpress epithelial (cytokeratin) and mesenchymal (vimentin) cytoskeletal markers. The tumor cells also display enhanced expression of desmosomal and tight-junction proteins. The apparent mesenchymal to epithelial conversion of the tumor cells mimics the conversion that occurs during embryonic kidney development, suggesting that Met-HGF/SF signaling plays a role in this process as well as in tumors that express both epithelial and mesenchymal markers.