Overexpression of Igbp1 and mEd2 impairs differentiation of I/11 erythroid progenitors. I/11 cells transduced with an empty vector (EV; A) or Igbp1 (B) or mEd2 (C) expression vectors were seeded in differentiation medium. Four single-cell–derived clones for each construct were analyzed at regular intervals during 4 days. Cumulative cell numbers (left panels) and hemoglobin content per cell volume (arbitrary units [a.u.], right panels) were measured. (D) At day 3, cells were harvested for cytospins and stained for hemoglobin (brown) and histologic dyes. Hemoglobinized and enucleated erythrocytes are abundant in control cells, whereas Igbp1- and mEd2-expressing cells contain mainly blasts.