CFSE is a fluorogenic reagent which is frequently used in cell labeling and cell proliferation procedures. Non-fluorescent and moderately hydrophobic in its diesterified form, the dye easily penetrates lipid bi-layers of living cells and is quickly converted to the amine-reactive, green fluorescent form (CSFE) upon cleavage of the acetate groups by intracellular esterases. This highly fluorescent form is membrane-impermeant and is retained on and within the cells, while any excess probe is easily washed away in subsequent wash steps. Because CFSE forms a strong bond inside the cell, it is retained within the cell indefinitely and is inherited by daughter cells. It will not be incorporated into adjacent cells.