New therapeutic strategies are required to overcome the limitations of conventional breast cancer treatment. Suicide gene therapy offers a potential approach to this type of tumour, since systems based on the use of prodrugs may present some drawbacks related to toxicity, drug release and bioavailability. The gef gene has cell-killing functions in Escherichia coli and does not depend on the use of a prodrug for its action, making it an attractive target for suicide gene therapy. We created a gef-overexpressing human breast cancer cell line (MCF-7TG) by transfecting the gef gene under the control of a pMAMneo promotor. Dexamethasone-induction of gef gene expression in MCF-7TG cells produced a significant decrease in Ki-67 expression, which is a known proliferation marker. In addition, annexin-V-FITC and propidium iodide assays showed the presence of apoptotic cell death, which was confirmed by scanning electron microscopy. The most significant finding was the presence of "craters" in the cell membrane, as previously described in other apoptotic breast cancer cells. These results demonstrate the ability of the gef gene to down regulate Ki-67 expression and induce apoptosis in a breast cancer cell line, suggesting its potential application as a new gene therapy strategy for this type of tumor.