Meis1a and Meis1b inhibit the in vivo erythroid potential of the MEP. Stem/progenitor cell-enriched BM cells from donor mice were transduced with a control vector (IRES-hCD4) or a retroviral construct expressing Meis1a (Meis1a-IRES-hCD4), Meis1b (Meis1b-IRES-hCD4), or Meis2(2b) (Meis2(2b)-IRES-hCD4) as indicated. The transduced BM progenitor cells were transplanted into sublethally irradiated recipients and BM was analyzed 4 to 6 weeks later. (A) BM was analyzed for expression of hCD4, B220, c-kit, CD71, and CD41. Shown are 2-color contours of CD71 and CD41 expression for cells gated as B220⁻ c-kit⁺. Numbers indicate the percentage of cells in the indicated gates. (B) The percentage of megakaryocytic progenitors (CD41⁺) or erythroid progenitors (CD71⁺) as in panel A was determined for both transduced (hCD4⁺) cells and nontransduced (hCD4⁻) cells within the same recipient. Shown is the ratio for each vector for megakaryocytic progenitors (CD41⁺) or erythroid progenitors (CD71⁺) as indicated. Each dot represents data from an individual recipient (n = 6). *P < .05. **P < .01. ***P < .001. (C) BM from recipients described in panel A was analyzed for expression of hCD4, B220, c-kit, Sca1, CD105, and CD150. Shown are 2-color contours of CD105 and CD150 expression for cells gated as B220⁻ c-kit⁺ Sca1⁻. (D) The percentage of pre-MegE and MkP (CD105⁻ CD150⁺), CFU-E (CD105⁺ CD150⁻), or granulocyte-macrophage progenitors and pre-GM (CD105⁻ CD150⁻) progenitors was determined for both the transduced (hCD4⁺) and nontransduced (hCD4⁻) BM cells within the same recipient. Shown is the ratio of hCD4⁺ to hCD4⁻ progenitors for each construct and cell population. Each dot represents data from one recipient mouse (n = 6). *P < .05. **P < .01. ***P < .001.