Effect of ATM inhibition on DNA damage foci repair Synchronized normal human diploid cells in G1 were electroporated with 100 U of various restriction endonucleases with or without 20 μM KU55933. The cells were incubated for the time indicated before fixation. At least 1000 nuclei were examined. C; mock-treated control cells without restriction endonucleases. Black bars and dark gray bars represent cells with Type IV and Type III nuclei, respectively.