FIG. 1.

Chk1/Chk2 regulates the initiation and maintenance of checkpoint arrest. (A) Checkpoint initiation requires ATM and Chk1/Chk2. Mitotic entry was examined in 1BR3 hTERT cells following 3 Gy IR in untreated cells or cells treated with the ATM inhibitor or Chk1/Chk2 inhibitor 30 min prior to IR. Cells, >400, were scored for mitotic index. Results represent the mitotic index relative to untreated cells. Maximal levels of p-Chk1 and p-Chk2 are reached within 30 min after 3 Gy. 1BR hTERT and A549 cells were exposed to 3 Gy IR, and p-Chk2 (B) and p-Chk1 (C) levels assessed by IF at various times post-IR. (D) Checkpoint maintenance requires Chk1/Chk2. The time of mitotic entry following exposure to 3 Gy IR in cells either untreated or treated with the Chk1/Chk2 inhibitor 30 min post-IR was examined. Error bars represent the standard error of the mean (SEM) from 3 experiments.