Strep•Tag® technology is based on the strong and specific interaction between biotin and streptavidin. To take advantage of this interaction for recombinant protein purification, the 8-amino acid Strep•Tag II fusion tag was developed to bind to the biotin binding pocket of streptavidin. Likewise, the Strep•Tactin protein is a streptavidin derivative developed for optimal Strep•Tag II binding. The binding affinity of Strep•Tag II for Strep•Tactin is approximately 100 times higher than for streptavidin. The Strep•Tactin family of products offers a variety of resins for rapid one-step affinity purification of proteins containing the Strep•Tag II fusion tag. The Strep•Tag II sequence binds to the Strep•Tactin, and after unbound proteins are washed away, the purified target protein is competitively eluted with 2.5 mM desthiobiotin, an analog of biotin that reversibly binds Strep•Tactin. The purification steps may be performed in PBS or other physiological buffers.