Streptavidin is a tetrameric protein purified from Streptomyces avidinii.  It has wide use in numerous molecular biological protocols dues to its strong affinity for biotin.  The original Strep-tag is a nine amino acid peptide with high specificity and affinity towards streptavidin.  The addition of this tag to the C-terminus of recombinant proteins allowed the simple purification of protein by use of affinity columns.  It was re-engineered (and re-named Strep-tag II) to allow it to also placed at the N-terminus of recombinant proteins.