Figure 4.

NBS and ATR-Seckel cells fail to arrest at the G2/M checkpoint following UV treatment. (A) The CBPI checkpoint assay. Cells were treated as indicated for 2 h followed by addition of cytochalasin B, an inhibitor of cytokinesis. After 72 h, cells were scored for the percentage of binucleate/mononucleate cells. The results represent the mean and s.d. of three experiments except for ATLD where n=1. Two experiments using LB195 cells gave similar results to those obtained with the LB112 cells. Results for LB197 (not shown) are identical to those obtained with GM2188. Black bars represent mononucleates and the white bar represents binucleates. (B) G2/M checkpoint arrest monitored by mitotic index. Cells were untreated or irradiated with 2.5 J m⁻², incubated for varying times up to 24 h in the presence of 1.5 μM nocodazole and the percentage of mitotic cells counted. The results shown are from a single experiment. For all lines, similar results were obtained in three experiments where the mitotic index was monitored at 24 h post-treatment.