Late G₂ checkpoint arrest is attenuated in RA5 and RG2 cells after IR. Cells were irradiated with indicated doses of γ-IR (A-D) or α-IR (E-H) and assayed for mitotic index as described in “Mitotic index assay.” Data are normalized to unirradiated controls and are mean ± SEM from 3 independent experiments. Cells were analyzed for both time responses following specific doses (A-C,E-G) and dose-response at 24 hours after IR. (D,H) Two-way, repeated measures ANOVA was used to test for differences at each dose. *P < .05. **P < .01.***P < .001.