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 （AWRHPQFGG） was a nine amino acid peptide with high specificity and affinity towards streptavidin which allows the simple purification of protein by use of affinity columns, but required addition to only the C-terminus of recombinant proteins. To also allow a Strep-tag to be placed at the N-terminus of recombinant proteins, it was re-engineered and re-named Strep-tag II.