Strep-tag is a widely used affinity tag in protein purification systems. It consists of two types Strep-tag II and Twin Strep-tag II. Strep-tag II is a short peptide tag consisting of 8 amino acids (WSHPQFEK) that can be fused to proteins as an N-terminal or a C-terminal tag with minimal effect on recombinant proteins. The further improved Twin Strep-tag II is a sequence of two Strep-tag II sequences in a sequential order (linked by internal amino acids), and this tag is capable of gentle and rapid purification like Strep-tag II. The two tags are free to bind either of the ligands in Streptavidin ST and STarm. Tag/ligand binding depends on the desired binding strength and application. The affinity of these two tags to Streptavidin ST and STarm is in the μg/ml range, a high affinity that is not achieved by any other existing affinity labeling system. In addition, this flexibility of combining tags and ligands allows purification of recombinant proteins under physiological conditions.