Absence or dominant inhibition of Runx1 reduces Cebpa expression. (A) Marrow mononuclear cells from Runx1(f/f) or Runx1(f/f);Mx1-Cre mice exposed to pIpC were subjected to Western blot analysis for C/EBPα or β-actin (left panel; representative of 3 experiments), and total cellular RNA from these cells were assessed for expression of Cebpa, Pu.1, or Runx1 mRNAs, relative to the RNA encoding ribosomal protein mS16, via quantitative RT-PCR (right graphs; mean and SE from 3 determinations). (B) Similar RNA analysis was conducted with lineage⁻ marrow cells from these mice. (C) Marrow cells from control or Runx1-deleted mice were sorted into HSCs, CMPs, GMPs, and MEPs. Total cellular RNAs from these populations were then analyzed for Cebpa, Pu.1, or Runx1 mRNA expression (mean and SE from 3 determinations). (D) RNA isolated from Runx1(f/f) marrow cells transduced with pBabePuro or pBabePuro-Cre, either immediately after puromycin selection (d0) or 3 days later (d3), were analyzed form Cebpa, Pu.1, and Runx1 expression (mean and SE from 3 determinations). (E) WT marrow cells transduced with RUNX1a or with the empty MIG retroviral vector were transplanted into irradiated, syngeneic recipients. Four weeks later, RNAs isolated from total BM or lineage-negative (lin⁻) marrow cells were analyzed for expression of Cebpa or Pu.1 relative to β-actin mRNA (mean and SE from 3 determinations).