endo-BCN-PEG4-NHS ester is a PEG derivative containing an NHS ester group and a BCN group. The NHS ester 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. PEG Linkers may be useful in the development of antibody drug conjugates and drug delivery methods.