N-(Azido-PEG2)-N-Boc-PEG3-NHS ester is a developed as branched labeling reagent. The azide group enables PEGylation via Click Chemistry. The protected amine can be deprotected under acidic conditions. The NHS ester can be used to label the primary amines (-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules.