To create an efficient detection and purification system based upon fusion polypeptides, several requirements must be met. One major demand on the added marker segment is that it should not interfere with the native folding of proteins to which it is attached. Secondly, the marker peptide sequence should be water-soluble and should retain a high degree of exposure on the surface of the protein, so that it can readily interact with its ligand. It should also be suitable for a mild and inexpensive affinity purification procedure. Finally, an easy removal of the marker peptide leading to a native product is also advantageous.