FIG. 2.

Atm and Brca1, but not Nbs1, are required for the IR-induced G₂ checkpoint. (A) Flow cytometric profiles of cell cycle distribution before [IR(−)] and 1 h after [IR(+)] IR using a combination of staining for DNA content (y axis) and for histone H3 phosphorylation (x axis). The 4N population of cells is distinguished as either in G₂ or in mitosis (encircled, M). Control cells (GM0536 and HeLa+vector) and cells defective in Nbs1 (NBS7078A) exhibit a marked decrease in the number of mitotic cells after IR, while cells defective in Atm (GM9607 and HeLa+kdATM) or Brca1 (HCC1937) continue to enter mitosis after IR. (B) Mitotic cells as a percentage of total cells before and after IR (quantitation of data shown in panel A). The error bars represent the variability after combining the results of at least three different experiments for each cell line.