AMCA-X, SE is one of the most popular blue fluorescent tagging molecules. It is widely used to label antibodies, proteins and small drug molecules. AMCA-X succinimidyl ester contains a seven-atom aminohexanoyl spacer, the so-called ‘X’ spacer, between the fluorophore and the reactive group. It is well known that the spacer between the fluorophore and the reactive group usually separates the fluorophore from the biomolecule to which it is conjugated, potentially reducing the quenching that typically occurs upon conjugation. In some cases, the ‘X’ spacer allows the dye to be more available for recognition by secondary detection reagents.