Transcriptional promoters responsive to low doses of X-irradiation may be useful in developing a new strategy in gene therapy combined with conventional radiotherapy. The retrovirus-mediated gene trap screening identified c-IAP2 as one of genes possessing such promoters. The analysis of the cis-elements responsive to X-irradiation in c-IAP2 promoter revealed that the NF-U UB binding sites were necessary and sufficient for the X-rayresponsiveness. We constructed the plasmid p4NFB-BAX, which had four tandem repeats of the NF-U UB binding sites of c-IAP2 promoter (4NFB) and a suicide gene BAX under the control of 4NFB. The human tumor cells transfected with p4NFB-BAX significantly reduced the number of cells that survived 2 Gy irradiation. ß 2001 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.