Fluorescent dye that is water-soluble and forms stable thioether bond with free thiol groups. Compared with Cy3 conjugates, those of Fluorescent Dye 555-I Maleimide are slightly red-shifted, resulting in an optimal match to filters designed for Cy3 dyes in accompany with improved photostability. Besides, their fluorescence is pH-independent from pH 3 to 11.