We have analysed the radiation response of a human medullary thyroid carcinoma cell line (MTT), characterized by the absence of a functional p53 protein, and the consequences of MDM2 overexpression in this process. We show that the product of the mdm2 proto-oncogene is able to sensitize MTT cells to ionizing radiation. After radiation treatment, MTT cells display histograms consistent with a G2M arrest. MTT cells expressing MDM2 (MTT-mdm2) are unable to respond to DNA damage with G2M arrest, and display a high percentage of apoptosis. MTT-mdm2 cells show high levels of E2F-1 protein, suggesting that the induction of apoptosis observed upon MDM2 overexpression could be dependent on E2F-1. This observation is further supported with assays showing that E2F-1 binding to specific DNA sequences is enhanced in MTT-mdm2 cells. Likewise, transactivation of reporter constructs exclusively dependent on E2F-1 is also elevated after transfection with MDM2. This effect can be reverted by transient transfection with p19ARF. To link the expression of E2F-1 with the induction of apoptosis, we generated clonal cell lines overexpressing E2F-1. Transfection with E2F-1 results in a low number of outgrowing colonies with reduced proliferation rates, indicating that E2F-1 is deleterious for cell growth. This negative regulation correlates with an increase in the percentage of the cell population with DNA content below 2N, suggesting that E2F-1 promotes apoptosis. Finally, overexpression of E2F-1 sensitizes MTT cells to radiation exposure. We conclude that the effects observed by MDM2 overexpression could be mediated by E2F-1.