The amine-reactive 7-hydroxycoumarin-3-carboxylic acid, succinimidyl ester can be used to create blue-fluorescent bioconjugates with excitation/emission maxima ~419/445 nm. Because the fluorescence may not be as bright as that of other dyes or may be obscured by autofluorescence, hydroxycoumarin derivatives are only recommended for use with highly abundant targets. Unlike AMCA and Alexa Fluor® 350 dye hydroxycoumarin exhibits pH-sensitive spectra.