At present the protein expression systems used commonly by researchers incorporate an affinity tail fused to the protein of interest. These affinity tails provide a convenient and efficient method for the purification of the expressed fusion protein using affinity chromatography. Many different affinity tails have been developed and a few of the commonly used fusion protein expression systems are listed in Table 1. The plasmid expression vectors for the production of fusion proteins in various hosts are available commercially and with the advent of the polymerase chain reaction (PCR) any gene of known sequence can be cloned into any expression vector. Most affinity tails are linked to the N-terminus of the protein of interest, but some affinity tails are able to be linked to either the N- or C-terminus of the protein of interest. The choice of affinity tail to use for the expression of any particular protein is empirical since the factors leading to the high expression of recombinant proteins in foreign hosts have yet to be elucidated. Table 1 Systems Commonly Used for the Expression of Recombinant Fusion Proteins in E. coli ( a ) Affinity tail Elution ligand Supplier References Glutathione S-transferase Glutathione Pharmacra (5) Transitional metals Transrtional metals Qiagen, Inc (6,7) Bmdmg polypepttdes e.g,Zn(2+),Cu(2+),N(1) (2+) Maltose bmdmg protein Maltose New England (8) Biolabs Staphylococcus aureus IgG Pharmacia (9) protein A Biotmylated pepttdes Streptavidin or Promega Promega avidin ( a )Transrtronal metal brndlng polypepttdes can be fused to either the N or C terminus of proteins.