Effect of telomerase deficiency on the distribution of senescence-related γ-foci and activation of damage signaling. (A) Distribution of γ-foci in low population doubling (PD) and pre-senescent mouse embryonic fibroblasts (MEFs) from second (TEG2) and fifth (TEG5) generation telomere reverse transcriptase (TERT)-null mice. (B) Distribution of γ-foci in splenic lymphocytes taken from young and old first (TRG1) generation mTR-null mice, and from young and old fifth (TRG5) generation mTR-null mice. Scoring is as in Figure 1B. On average more than 10 metaphases were screened per point in independent experiments. Error bars signify standard errors. (C) Levels of DNA damage checkpoint proteins in early and high PDs MEFs from second (TEG2) and fifth (TEG5) generation TERT-null mice. Relative induction in senescent compared with low PD cells of the proteins shown in the left panel. Dashed line denotes no induction.