Hela-cells transfected with GFP (green histogram) and non-transfected control cells (red histogram). About 50% of the cells in this examlpe was transfected.
We can sort the GFP positive cells, and the user can grow them for further studies. Usually the GFP is co-transfected with a special gene of interest.
It is also possible to transfect cells with other fluorescent proteins like YFP (Yellow fluorescent protein) and dsRed (double stranded Red). One can also transfect the cells to express an antigene on the cell surface which one can stain with different fluorochromes.