5-Carboxyfluorescein (5-FAM) is a fluorescent dye derived from fluorescein, characterized by a carboxylic acid functional group at the 5-position of the xanthene ring. It is a key intermediate for synthesizing more complex fluorescent probes and conjugates. The compound typically exists as a mixture of isomers and exhibits pH-dependent fluorescence, with strong emission in the green spectrum (around 520 nm) upon excitation at approximately 494 nm. Its primary application is as a fluorescent label and tracer in life science research. It is widely used to label peptides, proteins, and oligonucleotides, enabling detection in techniques such as fluorescence microscopy, flow cytometry, and fluorescence resonance energy transfer (FRET) assays. It serves as a common building block for synthesizing fluorescent dyes like FAM (6-carboxyfluorescein) isomers and other derivatives used in diagnostic kits and molecular biology. As a versatile fluorophore precursor, it is also employed in the development of sensors and in analytical chemistry for detection and quantification purposes. It is sensitive to photobleaching and its fluorescence intensity is quenched at low pH. Proper handling precautions are required as it may cause eye and skin irritation.