Linkers are the process of covalent binding between two biomolecules. Common linkers include amine coupling of amino acid residues in lysine, thiol coupling of cysteine residues, and frequently occurring photochemical free radical reactions. The product of biological coupling is a biological conjugate. The most common biological coupling is the coupling of small molecules, such as biotin to proteins. And the connection between proteins, such as the coupling of antibodies and enzymes. Other uncommon coupling molecules such as oligosaccharides, nucleic acids, synthetic polymers such as polyethylene glycol and carbon nanotubes. Bioconjugated drugs include types such as antibody drug complexes (ADC), polyethylene glycolated proteins, siRNA complexes, and vaccine complexes.