Streptavidin, a protein from Streptomyces avidinii, is known to bind to biotin with very high affinity (dissociation constant 10-14 M). It has a tetrameric structure in which each molecule of Streptavidin binds to four molecules of biotin. These interactions have been utilized to isolate variety of biotin labeled proteins, antibodies and other biomolecules (BioVision Cat #s 2322, 2323, 2326, 2345-2347). BioVision's leak-resistant Streptavidin conjugated beads are prepared by covalent conjugation of Streptavidin to highly cross-linked Sepharose beads by a proprietary method.