Cytotoxicity and radiosensitization of AA8 and irs1SF CHO cells with dFdCyd. Panel A: Sensitivity of AA8 (■) and irs1SF (▲) cells to dFdCyd was determined after a 4 h drug incubation, and survival was measured by a colony formation assay. Points, means of triplicate determinations ± SEM. Western blot demonstrates that irs1SF cells were deficient in XRCC3 conferring HR deficiency, whereas XRCC3 is present in AA8 cells. Radiosensitization with dFdCyd was determined in AA8 (panel B) and irs1SF (panel C) cells after incubation with either no drug (■), 0.5 μM (▼) or 5 μM (●) dFdCyd for 4 h, followed by irradiation at the indicated doses. Radiosensitization was measured using a colony formation assay with correction for the cytotoxicity of dFdCyd alone, as described in Materials and Methods.