Cy5.5-PEG-N3 (MW 5000) is a fluorescent azide dye utilized for the amino group modification in peptides, proteins, and oligonucleotides. As a reagent for click chemistry, it incorporates an azide group that can engage in copper-catalyzed azide-alkyne cycloaddition reactions (CuAAC) with alkyne-containing molecules. Additionally, it can participate in strain-promoted azide-alkyne cycloaddition (SPAAC) with molecules containing DBCO or BCN groups.