Nonclustered DNA damage and DSB repair are improved in RA5 and RG2 cells compared with HL60. Cells were irradiated with 2 Gy γ-IR (A) or an approximate dose of 0.5 Gy α-particles per nucleus (B) and assayed for percent foci-positive cells (n = 500) relative to controls at 30 minutes and 4 hours after IR. (C) Cells were irradiated with 2 Gy γ-IR on ice in PBS and immediately processed for DSB formation by the neutral comet assay. Data are box and whisker plots: whiskers are 1st and 99th percentiles; and +, means (calculated from at least 100 cells per sample). (D-F) Cells were irradiated with a test dose of 3 Gy of γ-rays and allowed to incubate under standard cell culture conditions for the indicated times before additional irradiation with indicated doses of γ-rays, and then assayed for clonogenic survival. Cells were irradiated under hypoxic conditions (< 0.1% O₂) with indicated doses of γ-rays before assaying for clonogenic survival (G). (H-I) Cells were treated with indicated doses of bleomycin (H) or neocarzinostatin (I) and assayed for apoptosis 48 hours after treatment. They were are fitted by nonlinear regression. IC₅₀ values exceed the 95% CI for both resistant clones. Data are mean ± SEM from at least 2 independent experiments. *P < .05. **P < .01. ***P < .001.