Trastuzumab is a humanized monoclonal antibody that binds to the extracellular domain IV of ERBB2 and thereby limits its activation.3 Pertuzumab, also a monoclonal antibody, disrupts ERBB2 dimerization via binding of the extracellular domain II of ERBB2, thereby inhibiting ligand-activated signaling with other growth factor receptors.4 Together, trastuzumab and pertuzumab synergistically inhibit tumor growth through their complementary mechanisms of action.4 This evidence concerns the gene ERBB2 and neoplasm.