Fluorescein-maleimide is a fluorescein dye conjugate containing a maleimide group. In the pH range of 6.5 to 7.5, the maleimide components in fluorescein-maleimide react with thiol-containing compounds or biomolecules to form thioester bonds. This compound is specifically used for the fluorescent labeling of proteins, nucleic acids, or other molecules that have one or more thiol groups.