The basic technology behind the Strep-tag System is a Two-Step Affinity Protein Purification using sequential purification of recombinant proteins carrying two affinity tags. Proteins that carry these two small tags (the 6xHis tag and the 8 amino acid containing Strep-tag) are efficiently expressed in E. coli, insect, or mammalian cells using pQE-TriSystem His·Strep vectors. After cell lysis and clearing of the lysate, proteins are initially purified by a procedure based on the proven 6xHis-tag-Ni-NTA interaction. After elution from the Ni-NTA matrix using imidazole, recombinant proteins (which also carry the Strep-tag epitope) are loaded directly onto a Strep-Tactin matrix (Strep-Tactin SuperFlow or Strep-Tactin Magnetic Beads). Protein is eluted from the Strep-Tactin matrix using either biotin or desthiobiotin. This two-step affinity purification delivers ultrapure (>98% pure) protein. The order of purifications can also be reversed (i.e., Strep-Tactin followed by Ni-NTA purification). Since purification is done under native conditions, the Two-Step Affinity Purification System can be beneficial for highly demanding applications (i.e., crystallization studies).