Thiol-reactive, cell permeant green fluorescent probe with high selectivity (Ex/Em: 492/517nm). This fluorescent dye is well suited for monitoring cell movement or location. Freely passes through cell membranes, where it is transformed into cell membrane-impermeant reaction products and is converted to the highly fluorescent probe (fluorescein) once the acetate groups are cleaved by intracellular esterases. The dye exhibits ideal tracking properties: it is stable, nontoxic at working concentrations, well retained in cells and brightly fluorescent at physiological pH. The green excitation/emission spectra are well separated from GFP (green fluorescent protein) and RFP (red fluorescent protein) spectra allowing for multiplexing.