During radiotherapy most cancer cells are removed, but some cells are not. These remained cancer cells become resistant to radiotherapy and lead to cancer recurrence. Radioresistnat cancer cells show different gene expression profile than radiosensitive tumor cells. Among the genes distinctly up or down-changed in expression level, some are associated with development of radioresistance. We used microarrays to select multiple genes showing distinct change in expression level for seeking genes that contribute to the develpoment of radioresistance in breast cancer cells during radiotherapy. Overall design: We selected 2 kinds of cell lines with different radiosensitivity for RNA extraction and hybridization on Affymetrix microarrays. To that end, parental MCF-7 cells were irradiated with 5 Gy twice a week and then cultured until survived cells recovered. The same cycle of irradiation were repeated until cells exhited distinct radioresistance. We tested cells' sensitivity to radiation by colony forming assay. Finally, MCF-7 and MR cells were selected as radiosensitive and radioresistant cells, respectively.