In bioconjugation, functional groups are used to link two or more molecules via covalent bonds. Functional reagents are used to introduce new reactive groups to target molecules or linkers. Despite the diversity of various molecules, four major chemical targets on target molecules account for most bioconjugation techniques: Primary amines (-NH2): can be crosslinked using NHS (N-hydroxysuccinimide) esters and water-soluble Sulfo-NHS esters. Carboxyl groups (-COOH): can be crosslinked to amino groups via peptide bonds. Thiol/sulfhydryl groups (-SH): can be linked together via disulfide bonds (-SS-). Aldehyde groups (-CHO): are usually generated in molecules by oxidation of carbohydrate groups, including glycoproteins, and can be crosslinked to molecules containing amino or hydrazine groups.