Loss of PSF1 leads to delay of HSC proliferation in the acute phase. (A) Percentage of KSL cells or HPCs (Lin⁻c-kit⁺Sca-1⁻) in the S/G₂/M phase among the total number of HSCs or HPCs, respectively, on day 6 after 5-FU injection as described in Figure 4A. ■ indicates HPCs; □, KSL cells. Mean values plus or minus SEM are shown (n = 5). *P < .05. (B) CFU-c assay using KSL cells obtained from mice on day 6 after 5-FU injection. +/+ indicates wild-type mice; and +/−, PSF1^+/− mice. ■ indicates CFU cluster (CLST; containing < 30 cells); □, CFU-C (CFU; containing > 30 cells). (C) Comparison between wild-type and PSF1^+/− KSL cells for sensitivity to 5-FU toxicity. Sorted KSL cells from the BM of 8-week-old mice were seeded in semisolid medium with indicated concentration of 5-FU, and total CFU-C number was counted after 10 days of culturing. Results are expressed as a percentage compared with control condition (100%). (D) Rescue experiments. PSF1^+/− mice were injected with 5-FU on day 0. One day after 5-FU injection (▲), Lin⁻CD45⁺c-kit⁺ cells (KL cells; 5 × 10⁴/mice) that had been derived from wild-type BM were injected into PSF1^+/− mice (n = 5).