Fig. 3. Time course of GFP expression of in differentiating Mixl1^GFP/w heterozygous and Mixl^GFP/GFP null ES cells as determined by flow cytometry. Both lines initiated GFP expression at day 2.5 and peak expression was seen at day 4. The frequency of GFP-positive cells diminished more quickly in Mixl1 heterozygous cells than in those deficient in Mixl1. The percentage of GFP-positive cells is shown in the lower right of each plot.