RA and RG clones have significantly reduced IR-induced apoptosis compared with HL60 cells after both γ-ray and α-particle exposure. Selected RA and RG cell clones and HL60 cells were irradiated with indicated doses of γ-IR (A,C) or α-IR (B,D), incubated 48 hours under standard cell culture conditions, and then assayed for apoptosis by annexin V and propidium iodide staining. Data are mean ± SEM from at least 3 independent experiments. Two-way, repeated-measures ANOVA with a Dunnett test showed all clones to be statistically different from HL60 at P = .05 level or better. (E) Data from a representative executioner caspase activity assay repeated 3 times at 24 hours after γ-IR. *P < .05, **P < .01, and ***P < .001, least significant of the 3 resistant clones at each dose.