A) HCT116 cells growing in chamber slides were exposed to 15 μM 5-FU (or vehicle) for 18 hours and 250 nM selumetinib (or vehicle) for 2 hours prior to irradiation with 4 Gy. Following irradiation, cells were washed with PBS, and media containing 250 nM selumetinib (or vehicle) was added. Cells were fixed at the time points indicated for immunocytochemical analysis of mitotic catastrophe. Nuclear fragmentation was evaluated in 150 cells per experiment. B) HCT 116 cells were treated with 5-FU (or vehicle) for 18 hours and selumetinib (or vehicle) for 2 hours prior to irradiation (4 Gy) and harvested at the specified times. Treated cell samples were added to a 150 μL staining solution (Guava Nexin Assay) containing 135 μL 1x apoptosis buffer, 10 μL Annexin V-PE, and 5 μL of 7-AAD. Samples (2,000 cells per sample) were evaluated by flow cytometry. Columns, mean; bars, standard deviation; *, p<0.05 as compared to other treatments at the specified time-point.