Pacific Orange™ succinimidyl ester is a violet-excitable (405 nm) dye with emission maximum at ∼551 nm. This allows it to be used with other violet-excitable fluorophores such as Pacific Blue™ dye for multiparameter analysis using a single excitation source. As a class of compounds, succinimidyl ester forms provide the most efficient and easy-to-use reaction chemistry to selectively link a dye to accessible primary amine groups on proteins, modified nucleic acids, or other biomolecules. Succinimidyl esters are excellent reagents for amine modification because the covalent bonds they form are as stable as the peptide bonds used to link amino acids in proteins. Pacific Orange™ succinimidyl ester is supplied as 1 mg of dry powder and should be stored at ≤-20°C, desiccated, and protected from light.