BCN reagent is a class of click chemistry reagents containing a very reactive BCN (bicyclo[6.1.0]nonyne) group, BCN reagents can react with azide-tagged molecules or biomolecules via copper-free Click Chemistry. It can be run either in aqueous buffers or in organic solvents depending on the property of the substrate molecules. Reagents with PEG arm will increases the compound's hydrophilicity. BCN reagents have become widely used in bioconjugation, labeling and chemical biology.