Distribution of cells with different nuclear type. Synchronized normal human diploid cells in G1 were electroporated with 100 U of various restriction endonucleases as described in METHODS. 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 represent cells without foci (Type IV). Dark gray and light gray bars represent cells with Type III and Type II nuclei, respectively. Off-white bars represent cells with Type I nuclei.