C/EBPβ is required for proliferation and differentiation of HSPCs at early and late phases of regeneration under stress conditions, respectively. (A) Experimental design for the BM replacement model, aimed at excluding nonhematopoietic differences between WT and Cebpb^−/− mice. Lethally irradiated WT mice (CD45.1⁺) were reconstituted with BM cells from WT or Cebpb^−/− mice (CD45.2⁺). After 12 weeks, the reconstituted animals were used in further experiments. (B) Cell-cycle analysis of LT-HSCs (CD150⁺CD48⁻ KSL cells) from WT- and Cebpb^−/−-reconstituted mice before and after 5-FU treatment. The frequency of cells in G0 phase (i), the frequency of cells in S-G2-M phase (ii), and the G1 phase/G0 phase ratio (iii) are shown (n = 4–5 per group and per time point in 2 independent experiments). (C) Numbers of LT-HSCs (CD150⁺CD48⁻ KSL cells), KSL (c-kit⁺Sca-1⁺ lineage^-) cells, and KL (c-kit⁺Sca-1⁻lineage⁻) cells per mouse at multiple time points after 5-FU administration (n = 4–5 per group and per time point in 2 independent experiments). (D) Representative flow cytometric patterns of subpopulations of KSL cells from WT- or Cebpb^−/−-reconstituted mice at days 10 and 18 after 5-FU administration. (i-iii) The frequencies of the indicated subpopulations are plotted (n = 5 per group and per time point in 2 independent experiments). Data are presented as means ± SD. *P < .05; **P < .01; and ***P < .001 (determined by the 2-tailed Student t test).