Increased spontaneous differentiation in Flk-1^–/– ES-EP cultures. (A) Outgrowth kinetics of wt and Flk-1^–/– ES-EPs derived from day 6 EBs are shown. (B) Hematologic and hemoglobin staining of wt and Flk-1^–/– ES-EPs under proliferation conditions (12 days in culture) is shown. Flk-1^–/– ES-EP cultures contain more differentiated erythroid cells compared with wt ES-EP cultures. Original magnification, × 100. (C) Under proliferation conditions, Flk-1^–/– ES-EP cultures produce more hemoglobin than wt ES-EPs. Error bars indicate standard deviation (mean ± SD).