Elevated DNA damage response in HSCs lacking Pot1b. (A) Colocalization of γ-H2Ax (green) and TRF1 (red) to telomeres in sorted LSK cells isolated from wild-type (WT) and Pot1b^Δ/Δ mice. Six 4-month-old mice per genotype were used for this experiment. (B) Quantification of colocalization of γ-H2Ax and TRF1 to telomeres in wild-type (WT) and Pot1b^Δ/Δ LSK cells. A total of 300 nuclei were scored per genotype. Error bars represent SEM. (C) BM metaphases isolated from Pot1b^Δ/Δ BM show chromosome end-to-end fusions (arrows), whereas wild-type (WT) metaphases show minimal abnormalities. (D) End-to-end chromosome fusions (arrows) and telomere signal-free ends (arrowheads) are elevated in Pot1b^Δ/Δ BM cells. Not all telomere-free chromosome ends are indicated. (E) Representative real-time PCR quantification of mRNA expression levels of p21, PUMA, and BAX in sorted wild-type (WT) and Pot1b^Δ/Δ LSK cells is shown. Each experiment was repeated in triplicate. Error bars represent SEM. (F) Histograms showing annexin V profiles of mice bone marrow LSK cells isolated from wild-type (WT) and Pot1b^Δ/Δ. Data from 6 mice per genotype are shown in the right panel (P = 2.0 × 10⁻⁴).