ERBB2 and neoplasm: A9B5−Bs−5 and A9B5−Bs−7 bound to cell-surface HER2 with high saturation and induced rapid HER2 internalization—an effect not observed with trastuzumab, pertuzumab, or T + P. Furthermore, both bpAbs exhibited superior or comparable growth-inhibition relative to T + P in both ligand-independent and ligand-driven tumor models.