Fluorescent dye that is water-soluble and forms stable thioether bond with free thiol groups. Fluorescent Dye 488-I Maleimide conjugates are significantly brighter than fluorescein conjugates and much more photostable. Additionally, the fluorescence of Fluorescent Dye 488-I Maleimide is not affected by pH between 4-10. Compared to Alexa Fluor 488, Fluorescent Dye 488-I Maleimide of single isomer has much hihger purity, bringing better performance consistancy from batch to batch.