G-418 drug resistance and GFP fluorescence were used to isolate the trapped ES clones whose trapped genes are down-regulated during ES cell differentiation. (A) Flow chart for the experimental procedure.(B) GFP FACS profiling of the ES clones. FACS plots of the ES clone 5B33 (left panel) and ES clone 5C1 (right panel) are shown with both the undifferentiated populations of cells (the green filled histograms) and differentiated populations of cells (the red unfilled histograms). Undifferentiated ES cells were grown in the presence of LIF and feeder fibroblast cells and the differentiated cells were grown without LIF and feeder fibroblast cells. "P" marks the GFP-positive population of ES cells present in the undifferentiated cells of the ES clone 5C1.