Fluorescent dye that is water-soluble and forms stable thioether bond with free thiol groups. Fluorescence emission of Fluorescent Dye 680-I Maleimide is well separated from that of other commonly used red fluorophores, such as TAMRA, Texas Red, Alexa Fluor 594, Fluorescent Dye 594-I, Fluorescent Dye 633-I, Fluorescent Dye 647-I and R-phycoerythrin, making it ideal for multi-color labeling. Besides, their fluorescence is pH-independent from pH 3 to 11.