Dominant inhibition of Runx1 by RUNX1a favors monopoiesis over granulopoiesis. (A) WT marrow cells transduced with RUNX1a or with the empty MIG retroviral vector were transplanted into irradiated, syngeneic recipients, and the proportion of marrow cells expressing GFP was determined 4 weeks later. The proportions of LSK, lin⁻c-kit⁻Sca-1⁺, lin⁻, B220⁺, Ter119⁺, Mac-1⁺Gr-1⁻, or Mac-1⁺Gr-1⁺ cells among the GFP⁺ cells were also assessed by FACS analysis (mean and SE from 3 determinations). (B) GFP⁺ cells were sorted and assessed for myeloid CFUs per 1E4 cells plated after culture in methylcellulose with IL-3, IL-6, and SCF (left; mean and SE from 3 determinations). The proportion of CFU-Gs relative to CFU-Gs plus CFU-Ms among these CFUs is also shown (right). (C) Cells isolated 4 weeks after transplantation were stained with propidium iodide, and the proportion of cells in the G₁, S, or G₂/M cell cycle phases was enumerated among GFP⁺ cells (mean and SE from 3 determinations).