ERBB2 and neoplasm: As multiple reports have indicated that pertuzumab inhibits tumor growth through blockade of HER2/HER3 dimerization and downstream oncogenic signaling, we speculated that synergy would be observed against HER2/HER3 expressing lines both in vitro and in vivo, while this synergy would not be observed in HER2+HER3– lines (16, 17).