The gef gene, found in Escherichia coli DNA, encodes a small (50 amino acids) protein which is related to cell-killing functions. We used the MS-36 melanoma cell line as an experimental model to examine the usefulness of the gef gene as a new strategy for cancer therapy. We transfected MS-36 cells using the pMAMneo vector, and induced gef gene expression with dexamethasone. This decreased the proliferation rate of MS-36TG by as much as 85% in comparison with MS-36 parental cells. The decrease in cell growth was accompanied with significant modifications of the cell cycle and morphology. The G1-phase gradually disappeared, with accumulation in the S-phase. However, studies with annexin V-FITC and 7-aminoactinomycin D failed to demonstrate induction of apoptosis. Morphological changes were an increase in cell size and the number of filopodia, and especially the appearance of pore-like alterations in the cell membrane which were not seen in parental cells. Our results demonstrate that the gef gene, a system independent of the administration of a prodrug, significantly reduces the proliferation of MS-36 cells. This gene may therefore be considered a new candidate for cancer gene therapy.