Fluorescein is a fluorophore commonly used to label proteins-protein-fluorescein conjugates are not usually susceptible to precipitation. In addition to its relatively high absorptivity, excellent fluorescence quantum yield and good water solubility, fluorescein has an excitation maximum of 494 nm that closely matches the 488 nm spectral line of the argon-ion laser, making it an important fluorophore for confocal laser-scanning microscopy and flow cytometry applications. Its fluorescence is pH sensitive and is significantly reduced below pH 7. Fluorescein emits most strongly between 500 and 550 nm, but it has a relatively broad emission spectrum reaching to over 600 nm. Several derivatives of fluorescein are commonly used, including FITC (fluorescein isothiocyanate), carboxylates and succinimidyl esters.