From a subtractive cDNA library, we isolated several cDNA clones which showed differential expression between highly and lowly metastatic human melanoma cell lines. One clone, designated nmb, showed preferential expression in the low-metastatic cell lines and was chosen for further characterization. Sequence analysis revealed that this clone represents a novel gene, encoding a putative transmembrane glycoprotein which showed the highest homology to the precursor of pMEL17, a melanocyte-specific protein. nmb RNA expression was absent in most tumor-cell lines tested and not restricted to the melanocytic lineage. Transfection of a partial nmb cDNA into a highly metastatic melanoma cell line (BLM) resulted, in 2 of 3 transfectants, in slower subcutaneous tumor growth and, in 1 of 3 transfectants, in reduction of the potential for spontaneous metastasis in nude mice.