Biotin, also known as vitamin B7, is a small biologically active molecule that acts as a coenzyme in living cells. It can be coupled to oligonucleotides at the terminal and internal position at multiple sites. Biotin labeling (also called biotinylation) is the process of covalently attaching biotin to a protein, nucleic acid, or other molecule, which has been widely used in numerous detection systems especially when the target of interest is expressed at low levels and cannot be detected by labeled antibodies alone.