The amine-reactive AMCA-X SE (6-((7-amino-4-methylcoumarin- 3-acetyl)amino)hexanoic acid, succinimidyl ester) and its conjugates yield blue-fluorescence (approximate excitation/emission maxima ~353/442) that can be used as a contrasting color in multicolor applications. Because its fluorescence may not be as bright as that of other dyes or may be obscured by autofluorescence, AMCA is only recommended for use with highly abundant targets. The succinimidyl ester includes and "X" or seven-atom aminohexanoyl spacer between the fluorophore and the reactive group to aid reactivity.