Trastuzumab (Herceptin®, Genentech Inc), is a humanized recombinant monoclonal antibody (mAb) directed against the extracellular domain of HER2/neu. Routine laboratory analysis of trastuzumab binding usually requires the expression of the HER2/neu antigen, which is a 185 kDa transmembrane glycoprotein, in mammalian cells. Because of its size and significant complexity, it is a difficult antigen to develop and produce. However, if we focus on the epitope, a peptide that mimics the binding site of trastuzumab can be designed instead of the complete protein. This would allow the use of a prokaryote expression system which has several advantages in productivity and costs. Here we show the development of a peptide that mimics the trastuzumab binding site on HER2/neu, a “mimotope” that can be expressed in Escherichia coli. We designed the vector with the sequence of the 13-residues peptide fused to Maltose Binding Protein (MBP), generated resistant clones and successfully induced the expression of the protein in a soluble form. Finally, we purified the fusion protein and verified by ELISA that can bind trastuzumab, but not other mAbs with different specificities. It is expected that this recombinant protein could be a useful tool for quantification of functional trastuzumab for development, quality control and clinical studies.