HSC exhaustion in the absence of Pot1b. (A) Serial peripheral blood count was used to monitor 2-month-old wild-type (WT) and Pot1b^Δ/Δ mice injected with a single dose of 5-FU (75 mg/kg, intraperitoneally). WBC counts are shown as a percentage of the initial baseline values for each group of mice (n = 6 mice per genotype, P = 7.8 × 10⁻³). Error bars represent SEM. (B) BM morphology (magnification ×20) before and 18 days after 5-FU injection for wild-type (WT) and Pot1b^Δ/Δ mice. (C) Representative FACs analysis of multilineage negative cells 12 days after 5-FU injection. Results are expressed as percentage of total nucleated BM cells for each fraction. (D) Survival outcome of sequentially 5-FU–injected wild-type (WT) and Pot1b^Δ/Δ mice. Results were analyzed with a log-rank nonparametric test and expressed as Kaplan-Meier survival curves (n = 7 mice per genotype, P = 2.3 × 10⁻³). (E) Representative cell-cycle analysis on LSK cells isolated from both wild-type (WT) and Pot1b^Δ/Δ mice. Data from the 6 experiments are shown in the right panel (P = 2.4 × 10⁻³). (F) LSK cells were stained for DNA (Hoechst 33 342) and RNA (Pyronin Y) to assess the relative proportions in the G₀, G₁ phases of the cell cycle. Data from 6 experiments are shown in the right panel (P = 1.0 × 10⁻⁴).