ERBB2 and breast cancer: Human epidermal growth factor receptor 2, which is involved in the regulation of cell growth and survival, is amplified in 15% to 20% of invasive breast carcinomas and confers shortened progression-free survival (PFS) and overall survival (OS) compared with other breast tumors.1 Activation of ERBB signaling is dependent on homodimerization or heterodimerization, with the ERBB2-ERBB3 dimer as the most potent inducer of cell proliferation.2 Clinical outcomes for patients with ERBB2-positive breast cancer have significantly improved with the development of ERBB2-targeted therapies.