Abstract: Resistance of glioblastoma multiforme (GBM) to radiation therapy poses a major challenge to its treatment. The intrinsic radioresistance of GBM has prompted research into radiosensitizers that could be used for therapeutic benefit. One of these is R115777, a farnesyltransferase inhibitor (FTI). Our data show that R115777 treatment inhibits cell growth in eight GBM cell lines. Those that undergo G1 or G2/M arrest in response to the drug are more sensitive to its cytotoxic effects than those that do not show arrest. R115777 also affects the radiation survival of the cell lines. All cell lines showed increased sensitivity if irradiated within 6 hours after drug treatment. However, for cells harboring wild type p53, radiosensitization faded after incubation times of longer than 8 hours. The kinetics of p21 induction correlated with the decreased radiosensitivity. To confirm the importance of the p53/p21 pathway in the development of resistance to R115777, we used HPV E6 to inhibit p53 and RNA inference (RNAi) to suppress p21 in U87 cells. Both methods maintained R115777 radiosensitization up to 24 hours. The rapidity by which R115777 radiosensitized cells led us to propose that a short-lived farnesylated protein, HDJ-2, could be a target for the drug. HDJ-2 is a co-chaperone of heat shock protein 70 (Hsp70), which could protect cells from radiation damage. Inhibition of farnesylation of HDJ-2 began within an hour of R115777 treatment. Genetic inhibition of HDJ-2 increased cellular radiosensitivity, while overexpression had the reverse effect. Furthermore, when cell were irradiated with 10 Gy, HDJ-2 migrated from the cytoplasm to the nucleus and this migration was inhibited by FTI. HDJ-2 may cooperate with other heat shock proteins to coordinate repair after radiation damage, although its role requires further elucidation. In conclusion, inhibition of protein farnesylation by R115777 can radiosensitize GBM cells, with probably several target proteins being involved. Interestingly, cells with wild type p53, including those in normal tissues, may be preferentially spared.