- Trastuzumab (Herceptin (Registered trademark)) is a recombinant humanized mouse monoclonal antibody (MAb) that binds to the extracellular (EC) domain of the HER2 receptor. Its clinical efficacy is limited to patients with 3+ HER2 tumor expression documented by immunohistochemistry (IHC) or a Vysis fluorescent in situ hybridization (FISH) ratio of greater than 2.2. IHC is a subjective measurement of HER2/neu protein while FISH is an objective measurement of amplification of the HER2 oncogene.