A variety of plasmids contain DNA that encodes an N-terminal tag consisting of six histidine (His) residues, followed by an extended multiple cloning sites. The 6xHis tag on the expressed recombinant proteins allows for efficient coupling to Ni++ affinity resins and purification by a single step chromatography. As is the case with other protein tags, this polyhistidine tag can be cleaved at sites recognized by enzymes such as thrombin and enterokinases to isolate the protein of interest.