C/EBPβ is required for stress-induced proliferation of HSPCs. (A) Schematic illustration of competitive transplantation to assess the initial expansion of HSPCs. A total of 1000 purified KSL (c-kit⁺Sca-1⁺lineage⁻) cells from WT or Cebpb^−/− mice (CD45.2⁺) and an equal number of WT KSL cells (CD45.1⁺), together with whole BM cells from CD45.1⁺/CD45.2⁺ WT mice, were transplanted into lethally irradiated WT recipients (CD45.1⁺/CD45.2⁺). (B) Representative flow cytometric analysis of KSL cells at 4 weeks after transplantation (i-ii). Frequency of test cells (CD45.2⁺) among KSL cells (iii) and the ratio of test cells (CD45.2⁺) to competitor cells (CD45.1⁺) among KSL cells (iv) are shown (n = 4 per group, representative of 3 independent experiments). (C) Cell-cycle statuses of WT and Cebpb^−/− LT-HSCs (CD150⁺CD48⁻ KSL cells) were analyzed at steady state and 36 hours after transplantation of 5 × 10⁶ BM cells into lethally irradiated WT recipients. (D) Representative cell-cycle statuses of LT-HSCs at steady state (i-ii) and 36 hours after BM transplantation (BMT) (iii-iv). (E) Percentages of LT-HSCs in each cell-cycle state at steady state (i) and 36 hours after BMT (ii) (n = 4 per genotype in 2 independent experiments). Data are presented as means ± SD. *P < .05; ***P < .001 (determined by the 2-tailed Student t test).