Fli-1 gene deletion drastically impairs the production of large mature megakaryocytes. Cre⁺ or Cre⁻ MEPs have been cultured for 5 days in liquid medium supplemented either with 10% FCS and a complete cocktail of myeloid cytokines or in serum-free medium containing 15% BIT and only cytokines favoring megakaryocytic differentiation. (A) Picture showing the drastic decrease in the number of large megakaryocytes in Cre⁺ MEP cultures. (B) Dot plot showing the decrease in the number of large mature megakaryocytes in Cre⁺ versus Cre⁻ MEP cultures according to increasing Fli-1 gene deletion efficiency. Relative numbers of large mature megakaryocytes were determined by counting at least 3 different fields of MGG stained cytospins obtained with equivalent numbers of cells from Cre⁺ and Cre⁻ MEP cultures (compilation of the results from 4 different couples of Cre⁺ and Cre⁻ mice and MEP cultures performed in either FCS containing or serum-free medium). (C) Single-cell RT-PCR analyses showing the proportion of large megakaryocytes still expressing Fli-1 mRNA (undeleted cells) isolated from culture of Cre⁻ MEP (lanes 1-10), from culture of Cre⁺ MEPs with an initial deletion efficiency of 85% (lanes 11-21) and from bone marrow of injected Cre⁺ mice displaying deletion efficiency in total BMC population of 55% (lanes 22-27). (D) Dot plot showing the decrease in the number of large mature acetylycholinesterase-positive megakaryocytes counted on BMC cytospins from 4 injected Cre⁺ mice displaying the indicated Fli-1 gene deletion efficiencies.