Avidin is a tetrameric protein originally isolated from chicken egg white evincing high binding affinity to biotin (Vitamin B7, earlier Vitamin H). Streptavidin on the other hand is of bacterial origin and has been isolated from bacterium Streptomyces avidinii. Similarly to avidin, streptavidin also shows high affinity to biotin, although both proteins share only about 30% homology. Despite their very similar secondary, tertiary and quaternary structure, both avidin and streptavidin vary from each other. Avidin is a larger, glycosylated molecule, which may cause nonspecific binding, especially with lectins. At a neutral pH it carries a positive charge, which may interact with negatively charged molecules such as DNA in the nucleus.