FIGURE 1: Fractionated radiation promotes acquired radioresistance in GBM cell lines. (A) Clonogenic survival curves for radiosensitive (U251, D456, and Jx39) and isogenic radioresistant (U251-RR, D456-RR and Jx39-RR) cell lines. Cells were irradiated with 2, 4, 6, or 8 Gy and immediately plated. Clonogenic survival was estimated on day 14 after irradiation. (B) Quantification of post-irradiation DNA damage in the radiosensitive and radioresistant GBM cells, assessed by single-cell gel electrophoresis assay under neutral conditions (neutral comet assay). Data are presented as the mean ± SEM (n=3). p < 0.01 (**), p < 0.001 (***), and p < 0.0001 (****), calculated by Student t-test. ns, not significant.