Fluorescein 5-Maleimide is widely used in biochemical and cellular research for labeling proteins, peptides, and other biomolecules. Its maleimide group specifically reacts with thiol groups (-SH) in cysteine residues, enabling precise tagging of target molecules. This property makes it valuable in fluorescence microscopy, flow cytometry, and other imaging techniques to study protein localization, interactions, and dynamics in live or fixed cells. It is also employed in the development of fluorescent probes and assays for detecting enzymatic activity or monitoring cellular processes. Additionally, it serves as a tool in drug discovery and diagnostics, where labeled biomolecules are used to screen compounds or detect biomarkers.