Runx1 and Cbfβ hypomorph results in HSC expansion. (A) Endogenous Runx1 and Cbfβ proteins were determined by western blotting in LSK populations selected by AutoMac from wild-type or Runx1^+/−Cbfβ^+/− mice. (B) Flow cytometric analysis of HSC and HPC compartments in WT and Runx1^+/−Cbfβ^+/− mice. n = 4 mice/group. (C) Summary of HSC (Lin⁻c-Kit⁺Sca-1⁺) and HPC (Lin⁻c-Kit⁺Sca-1⁻) population percentages in Lin⁻ populations in the genotypes described in panels A and B. (D) Summary of GMP (Lin⁻c-Kit⁺Sca1⁻CD34⁺CD16/32⁺), CMP (Lin⁻c-Kit⁺Sca1⁻CD34⁺CD16/32⁻), and MEP (Lin⁻c-Kit⁺Sca1⁻CD34⁻CD16/32⁻) percentages in HPC (lin⁻c-Kit⁺Sca-1⁻) populations in the genotypes described in panels A and B. (E) BM cells from WT or Runx1^+/−Cbfβ^+/− mice were transplanted into lethally irradiated mouse recipients. The spleens from each group were harvested and homogenized 8 or 12 days after transplantation and placed in Tellesniczky fixative to visualize the colonies. (F) The fractions of WT or Runx1^+/−Cbfβ^+/− (CD45.2⁺) BM cells were transplanted into lethally irradiated CD45.1⁺/CD45.2⁺ WT recipient mice along with 1 × 10⁵ WT (CD45.1⁺) helper cells. Engraftment was assessed 16 weeks after transplantation. The mean ± standard deviation percentage of donor-derived wild-type or Runx1^+/−Cbfβ^+/− (CD45.2⁺) in recipients’ BM is shown. APC-A, Anti-c-kit-APC; PE-A, Anti-Sca1-PE.