Pertuzumab is a recombinant humanized monoclonal antibody targeting the extracellular dimerization domain of Her2. This inhibits Her2 dimerization with Her1, Her3, and Her4, and downstream activation of the PI3K and MAPK pathways. Specifically, pertuzumab counters increased Her2 and Her3 dimerization signaling associated with trastuzumab resistance and is used in combination with trastuzumab and chemotherapy. Pertuzumab is approved for the first line treatment of Her2 overexpressing metastatic breast cancer and for neoadjuvant and adjuvant treatment of Her2 positive breast cancer.