Recombinant DNA technology allows the addition of short pieces of well-defined tags, "peptides" or proteins at the amino or c- terminus of target genes, which can provide 'affinity handles' designed to bind specific matrices. Therefore, tags enables a selective identification and purification of the protein of interest. The addition of a maltose binding protein (MBP) tag creates a stable fusion product that does not appear to interfere with the activity of the protein or with the cellular localization of the MBP- tagged product. The expression of polypeptides in-frame with maltose binding protein (MBP) allows for their easy, single- step purification from bacterial extracts under mild conditions using amylose resin. This system utilize a specific protease digestion site to facilitate correct cleavage of the fusion protein. Thus, the MBP system incorporates a factor Xa cleavage site at the carboxy terminus of the MBP sequence and cleavage by factor Xa separates MBP from its fusion protein. Many recombinant proteins have been engineered with MBP tags to facilitate the detection, isolation and purification of these proteins. Ant-MBP may be used in various immunoassay to identify the expression of a MBP fusion protein.