Tetramethylrhodamine-5-(and-6)-maleimide is widely used as a fluorescent labeling reagent in biochemical and cellular research. Its primary application is in the labeling of thiol-containing biomolecules, such as cysteine residues in proteins, due to its maleimide group's high specificity for thiols. This labeling allows for the visualization and tracking of proteins in live cells or in vitro experiments using fluorescence microscopy. It is particularly useful in studying protein-protein interactions, cellular localization, and dynamics. The bright red fluorescence emitted by the tetramethylrhodamine moiety provides excellent contrast and sensitivity, making it a valuable tool in fluorescence-based assays and imaging techniques. Additionally, it is employed in flow cytometry and fluorescence resonance energy transfer (FRET) studies to investigate molecular interactions and conformational changes in real-time.