Wikipedia
Strep-tag
The Strep-tag system is a method which allows the purification and detection of proteins by affinity chromatography. The Strep-tag is a synthetic peptide consisting of eight amino acids ( Trp- Ser- His- Pro- Gln- Phe- Glu- Lys). This peptide sequence exhibits intrinsic affinity towards Strep-Tactin, a specifically engineered streptavidin and can be N- or C- terminally fused to recombinant proteins. By exploiting the highly specific interaction, Strep-tagged proteins can be isolated in one step from crude cell lysates. Because the Strep-tag elutes under gentle, physiological conditions it is especially suited for generation of functional proteins.