The present invention relates to the cloning and expression of foreign protein or polypeptides in bacteria, such as Escherichia coli. In particular, this invention relates to expression tools comprising a FKBP-type peptidyl prolyl isomerase selected from the group consisting of FkpA, SlyD, and trigger factor; methods of recombinant protein expression, the recombinant polypeptides thus obtained, as well as to the use of such polypeptides.