Human melanoma cells growth arrest irreversibly, lose tumorigenic potential and terminally di erentiate after treatment with a combination of ®broblast interferon (IFN-b) and the protein kinase C activator mezerein (MEZ). Applying subtraction hybridization to this model di erentiation system permitted cloning of melanoma di erentiation associated gene-7, mda-7. Expression of mda-7 inversely correlates with melanoma development and progression, with elevated expression in normal melanocytes and nevi and increasingly reduced expression in radial growth phase, vertical growth phase and metastatic melanoma. When expressed by means of a replication incompetent adenovirus (Ad.mda-7) growth of melanoma, but not normal early passage or immortal human melanocytes, is dramatically suppressed and cells undergo programmed cell death (apoptosis). Infection of metastatic melanoma cells with Ad.mda-7 results in an increase in cells in the G 2 /M phase of the cell cycle and changes in the ratio of pro-apoptotic (BAX, BAK) to anti-apoptotic (BCL-2, BCL-XL) proteins. Ad.mda-7 infection results in a temporal increase in mda-7 mRNA and intracellular MDA-7 protein in most of the melanocyte/melanoma cell lines and secretion of MDA-7 protein is readily detected following Ad.mda-7 infection of both melanocytes and melanoma cells. The present studies document a di erential response of melanocytes versus melanoma cells to ectopic expression of mda-7 and support future applications of mda-7 for the genebased therapy of metastatic melanoma.