Cy5-PEG-N3 (MW 3400) is a Cy5 azide fluorescent dye designed for use with amino groups in peptides, proteins, and oligonucleotides. Serving as a click chemistry reagent, it features an azide group that participates in copper-catalyzed azide-alkyne cycloaddition (CuAAC) with alkyne-containing molecules. Additionally, it undergoes strain-promoted azide-alkyne cycloaddition (SPAAC) with molecules having DBCO or BCN groups.