Fli-1 gene deletion decreases the size and enhances the differentiation of MEP-derived erythrocytic colonies. Equivalent numbers of bone marrow MEPs purified from injected Cre⁻ or Cre⁺ mice have been cultured for 1 week in methylcellulose-medium containing a complete cocktail of cytokines (IL3, IL11, TPO, EPO, SCF, GM-CSF, Flt3-l). (A) Left part, light microscopy view of typical large and small erythrocytic colonies obtained; right part, histogram showing the relative proportions of large and small erythrocytic colonies generated by Cre⁻ or Cre⁺ MEPs (means and SDs from 3 different couples of fli1^fl/fli1^Δ Cre⁺ and Cre⁻ mice). Statistically significant difference is indicated by asterisk. (B) Representative field of May-Grünwald-Giemsa–stained cytospins from pooled Cre⁻ or Cre⁺ colonies at 5× magnification (left) and 20× magnification (right). Note the increased proportion of small acidophilic mature cells versus large basophilic immature erythrocytic cells in Cre⁺ compared with Cre⁻ colonies.