Background: Being able to predict the response of tumors to radiation therapy would improve the decision-making process involved in choosing treatment options for cancer. Expression of certain oncogenes and/or inactivation of tumor suppressor genes has been shown to alter cellular radiation responses; however, it is still not clear what marker or combination of markers would best indicate a radioresistant tumor, or whether such screening would be clinically useful. Current choices of markers are derived mainly from in vitro studies on cell survival after irradiation. In general, expression of transforming oncogenes increases cellular radioresistance. This was also demonstrated in this study for v-abl, bcr abl, v-Ha-ras, v-mos, and v-fes expressed in rat-1 cells. There are, however, conflicting data. Some of the discrepancies may in part be due to interactions between the oncogene-activated signals and other intrinsic or activated pathways. One downstream pathway that is required for oncogene-induced transformation involves c-myc. There is evidence that in some systems myc expression can potentiate ras-induced radiation resistance. Myc may therefore play an important role in determining tumor radioresistancy in the context of other oncogenes. Methods and Results: In this study, the role of c-myc in modulating intrinsic and oncogene-induced cellular radiation responses was investigated in more detail. Retroviral vectors were used to express c-myc and dominant negative mutant c-myc genes in rat 1 cells, with and without ca-transfection of v-abl as measured by clonogenic assay, rat 1. Cells infected with c-myc or v-abl were more resistant to irradiation than neo-transfected cells or control cells; however, cells doubly infected were not resistant, even though they had an increased transformation index. This indicates that transformation-related events per se do not necessarily lead to radiation resistance. It also suggests that the effects of c-myc on radioresistance may depend on what other pathways are activated. This conclusion was strengthened by the finding that expression of a dominant negative c-myc (dn-myc) mutant gene blocked v-abl-induced radiation resistance, but on its own made rat-1 cells more resistant to radiation. Conclusions: The apparently contradictory effects of c-myc in either enhancing or reducing radioresponsiveness may be explained by the dualistic roles of c-myc in promoting signal transduction pathways resulting in either cell proliferation or death, depending on what other pathways are activated. The studies indicate that it will be difficult to predict tumor response to radiation purely by examining expression of transforming oncogenes and it is likely that a number of markers will need to be examined to derive a reliable indication of tumor radiation response.