To track protein expression, localization, or conformational changes as components of cellular signaling pathways, biologists need general tools for the in vivo site-specific labeling of proteins with fluorophores or other useful probes. Fluorescein and its derivatives have been the most popular fluorescent derivatization reagents for covalently labeling proteins because of their relatively high absorptivities, excellent fluorescence quantum yields and high water solubilities. In addition, fluorescein-protein conjugates are usually not susceptible to precipitation. Fluorescein has an excitation maximum that closely matches the 488 nm spectral line of the argon ion laser, making it the predominant fluorophore for confocal laser scanning microscopy and flow cytometry applications. Many widespread optical filter sets are designed to excite efficiently and detect fluorescein's fluorescence. Fluorescein can most commonly be introduced into a protein through in vitro modifications with suitable amine-reactive reagents, such as N-hydroxysuccinimidylfluorescein.