Defective repair of γIR-induced DNA damage in RB1 knockout cells. (A and B) U2OS cells were treated with 2 Gy γIR, fixed 1 h (A) or 24 h (B) after treatment, and stained for γH2AX. Three clones per genotype were used, and γH2AX foci were quantified. The proportions of cells with discrete numbers of foci are shown as histograms, while the cumulative frequency of foci is shown in the inset. Differences in the distributions of foci were determined using the Kolmogorov-Smirnov test. A green asterisk indicates that RB1−/− is statistically different from the other genotypes, while a black asterisk indicates that all genotypes are statistically significantly different from each other. (C and D) H460 cells were treated with 1 Gy γIR, fixed 1 h (C) or 24 h (D) after treatment, and stained for γH2AX. (E and F) H1792 cells were treated with 1 Gy γIR, fixed 1 h (E) or 24 h (F) after treatment, and stained for γH2AX. All error bars are +1 SEM. *, P < 0.05.