Frequencies of spontaneous γ-H2AX positive foci and TIFs in "younger" and "older" cells. A and B. Average γ-H2AX positive foci in each cell line. C and D. Average TIFs per in each cell line. E and F. Mean frequencies of TIFs from two Artemis defective cell lines pooled together. All results are based on four independent experiments with an average of 200 cell nuclei scored per experiments per cell line.