Percentages of apoptotic cells in MGMT-unmethylated LN18 (a) and MGMT-methylated U87 cells (b) up to 72 h after irradiation. Cells were exposed for 5 h to 5 μM ABT-888 before being treated with 5 (b) or 10 (a) μM TMZ for 2 h and irradiated with 3 Gy X-rays. Dose–response percentages of apoptotic cells in MGMT-unmethylated LN18 (c, d) and MGMT-methylated U87 cells (e, f). Cells were exposed to 2, 3 and 4 Gy X-rays and fixed at 24 and 48 h after irradiation. Error bars indicate the standard error of two independent experiments. Please note the scale change on the Y-axis of the graphs. P values relatively to the control are also shown.