The megakaryocyte-erythroid progenitor (MEP) compartment is expanded in trisomy 21 fetal hematopoiesis. (A) Representative flow cytometric analysis of control (C) and trisomy 21 (T21) fetal liver MNCs. CD34⁺38⁺ cells were fractionated according to IL-3Rα and CD45RA expression to delineate different progenitor compartments: common myeloid progenitor (CMP, IL-3Rα^lo/CD45RA⁻, top left panel), granulocyte macrophage progenitor (GMP, IL-3Rα^lo/CD45RA⁺, top right panel), and megakaryocyte erythroid progenitor (MEP, IL-3Rα⁻/CD45RA⁻, bottom left panel). The numbers indicate percentages of each population within the CD34⁺CD38⁺ fraction. (B) Distribution of multipotential progenitors in T21 (n = 4) and C (n = 5) fetal liver MNCs. Data are plotted as percent of the CD34⁺CD38⁺ population. Bars represent the mean plus or minus SD. *P = .01, **P = .0003. (C) Relative expression of selected chromosome 21–encoded hematopoietic transcription factors in purified T21 (n = 3) and C (n = 4) fetal liver CMPs. Results shown are the mean plus or minus SD. *P = .03.