ATR inhibition increases radiosensitivity, irrespective of p53 status. (A) Survival of HCT116, U2OS or MCF10A cells exposed to increasing doses of irradiation in the presence of 1 µM VE-821 (open circles, dashed line) or a vehicle control of 0.1% DMSO (filled circles, solid line) for 24 h, then reseeded at low density for colony formation. Data are mean ± standard deviation from three separate experiments; (B) the surviving fraction at 2 Gy and 4 Gy was determined for paired and unpaired cell lines and the fold-sensitisation by VE-821 calculated from the ratio of the survival in the presence and absence of VE-821 for each independent experiment. HCT = HCT116, U2 = U2OS, M7 = MCF7, 231 = MDA-MB231, and M10A = MCF10A.