Maltose binding protein (MBP) is a useful affinity tag that can increase the expression level and solubility of the MBP tagged protein. It promotes proper folding of the fusion protein, and it can be also used to prevent an insoluble form (inclusion bodies). MBP is used to increase the solubility of recombinant proteins expressed in E. coli. In these systems, the protein of interest is often expressed as a MBP-fusion protein, preventing aggregation of the protein of interest.