Fluorescein is a fluorophore commonly used in microscopy, flow cytometry forensics, serology to detect latent blood stains, and in dye tracing (attached to certain biologically active molecules). This fluorescent dye/probe has spectral data of lambdaex 490 nm| lambdaem 514 nm in 0.1 M Tris pH 8.0 and is strongly pH dependent. It has an isosbestic point (equal absorption for all pH values) at 460 nm. Fluorescein derivatives are used in different applications in cell biology.