Gfi1 and Gfi1b are RUNX1 transcriptional targets in the early stages of blood development. (A) Semiquantitative RT-PCR analysis of Runx1, Gfi1, Gfi1b expression during blast colony development with Runx1^+/− and Runx1^−/− cells. β actin is the loading control. (B) Chromatin immunoprecipitation analysis showing the direct binding of RUNX1 on Gfi1 and Gfi1b enhancers. (C-D) Day 3.5 Runx1^−/− FLK1⁺ cells were transduced with the indicated retroviruses. Four days after culture, (C) the number of colonies was counted and (D) semiquantitative RT-PCR analysis of expression of the indicated genes was performed. β actin is the loading control. Values in the histogram correspond to an average of 3 transductions for each construct; the error bars correspond to the standard deviation.