ERBB2 and neoplasm: Pertuzumab is a humanized monoclonal antibody that binds to subdomain II of the HER2 receptor and blocks heterodimerization and signal transduction via MAPK (mitogen-activated protein kinase) and PI3K (phosphoinositide 3-kinase)/protein kinase-B (AKT) pathways essential for tumor growth (in contrast to trastuzumab, which binds to subdomain IV and does not inhibit heterodimerization of HER2/HER3 and HER2/HER1).