Classification of the nuclei with foci. Synchronized normal human diploid cells in G1 were electroporated with Pvu II (100 U) as described in METHODS, and they were incubated for 2 hours before fixation. The nuclei were classified into 4 types. Type I; nuclei with diffused phosphorylated ATM signal, but without 53 BP1 foci. Type II; nuclei with numerous foci more than 30 foci. Type III; nuclei with countable foci below 30. Type IV; nuclei without detectable foci.