Biotin-PEG-azide (MW 3400) is a biotin labeled PEG derivative. Biotin is an enzyme co-factor, can be used for labeling protein; PEG is a hydrophilic and water-soluble polymer with low toxicity; azide, is a moderately good leaving group, can react with alkyne by Cu-catalyzation, which improve the efficiency of biotin binding targets. It contains an azide group and can undergo copper-catalyzed azide-alkyne cycloaddition (CuAAc) with molecules containing alkyne groups. It can also undergo ring strain-promoted alkyne-azide cycloaddition (SPAAC) with molecules containing DBCO or BCN groups.