5-FAM (5-Carboxyfluorescein) (5-Carboxyfluorescein) contains a carboxylic acid, which can be used to react with primary amines via carbodiimide activation of the carboxylic acid. Fluorescein is the most common fluorescent derivatization reagent for labeling biomolecules. In addition to its excellent fluorescence quantum yield, relatively high absorptivity, and good water solubility, it also has an excitation maximum that closely matches the 488 nm spectral line of the argon-ion laser.