StreptavidinisatetramericproteinpurifiedfromStreptomycesavidinii.IthaswideuseinnumerousmolecularBIOLOGicalprotocolsduestoitsstrongaffinityforbiotin.TheoriginalStrep-tag(AWRHPQFGG)wasanineaminoacidpeptidewithhighspecificityandaffinitytowardsstreptavidinwhichallowsthesimplepurificationofproteinbyuseofaffinitycolumns,butrequiredadditiontoonlytheC-terminusofrecombinantproteins.ToalsoallowaStrep-tagtobeplacedattheN-terminusofrecombinantproteins,itwasre-engineeredandre-namedStrep-tagII.