(+)-Biotin possesses one of the strongest non-covalent interactions found in nature to streptavidin (Kd = 10-14 mol/l) and is relatively small in size, making it unlikely to affect the dynamics of the protein of choice. These two factors make (+)-Biotin a popular molecular probe. With a choice between hydrophilic PEG linkers and hydrophobic LC linkers, and a variety of functional group ends tailored to the chemical modification of choice, these biotinlyation reagents can be used to label and modify a variety of substrates.