Fluorescein-5-maleimide (N-(5-Fluoresceinyl)maleimide) is a fluorescent dye. Fluorescein-5-maleimide can be used to detect the redox state of thiols in eukaryotic cells. Fluorescein-5-maleimide can label peptides and is used to detect negatively charged nanoparticles. Fluorescein-5-maleimide can also label actin to explore its interaction with cardiac myosin-binding protein C (cMyBP-C), which helps in developing small molecule modulators for heart failure. Fluorescein-5-maleimide can screen mutant proteins that contain cysteine residues. The excitation wavelength of Fluorescein-5-maleimide is 494 nm, and the emission wavelength is 519 nm.