Radiation response of MCF-7 monolayer and mammospheres cells. (a) Clonogenic survival assay - MCF-7 monolayer and mammospheres were trypsinized, irradiated as single-cell suspensions with increasing doses of 60Co γ-radiation, and then plated. After18 days of incubation, the colonies (consisting of more than 30 cells) were fixed, stained with crystal violet and counted. (b) Cell proliferation assay - cells were plated in a 96-well plate, irradiated and incubated for five days. Cells were exposed to 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS, inner salt) for four hours and absorbance was measured at 490 nm. Absorbance was normalized to unirradiated controls. Error bars represent the mean ± standard error of the mean from three independent experiments.