The effect of ABT-263 is reduced for cell lines with limited radio-induced senescence. HCT-116 and KP4 cells were irradiated with 3 and 5 Gy X-rays with or without 0.5 µM of Olaparib. The medium was replaced 22 h after irradiation, and 1 µM of ABT-263 was added. (a,d) Quantification of SA-β-Gal positive cells 6 days after irradiation ((a): HCT-116; (d): KP4). (b,e) CDKNA1 mRNA level 3 days after treatment ((b): HCT-116; (e): KP4). (c,f) Quantification of EdU incorporation after a 10 µM 8 h pulse ((c): HCT-116; (f): KP4). At least three independent experiments were performed, and data are presented as mean ± 1 SD. Unpaired t-tests were performed (*: p < 0.05; **: p < 0.01) for CTL vs. ABT, CTL vs. Ola and Ola vs. Ola + ABT.