endo-BCN-PEG2-PFP ester is a click chemistry active ester. The PFP esters have similar applications as the NHS esters, but are more stable in aqueous solution. They can be used to label the primary amines (-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules. The BCN group can react with azide-tagged biomolecules.