SIRT1 deficiency leads to increased telomere damage in primary MEFs. (A) Representative images of TRF1 (red) and γ-H2AX (green) immunofluorescence. Colocalization events indicate occurrence of TIFs (yellow arrowheads). (B) Quantification of cells with more than three TIFs in primary MEFs of the indicated genotypes. Number of TIF-positive, total cells, and independent MEFs per genotype are shown. Fisher exact test was used for statistical analysis, and p-values are indicated. Error bars represent SEM. Bars, 10 µm.