ERBB2 and breast cancer: HER2 is a compelling therapeutic target in patients with breast6,8-10 and gastric or gastroesophageal junction (GEJ) cancers.11 For HER2-overexpressing or HER2-amplified breast cancer, several HER2-targeted therapies are approved for use in the adjuvant and metastatic settings, including trastuzumab (metastatic and adjuvant), pertuzumab (metastatic and adjuvant), lapatinib (metastatic), ado-trastuzumab emtansine (metastatic), and neratinib (adjuvant).