A human monoclonal antibody directed against the human epidermal growth factor receptor HER3 (ErbB3) with potential antineoplastic activity. Anti-HER3 monoclonal antibody LJM716 possesses a novel mechanism of action; LJM716 binds to and locks HER3 in the inactive conformation and does not interfere with its interaction with neuregulin (NRG). The inactivated form of HER3 blocks the PI3K/Akt signaling pathway, thereby inhibiting cellular proliferation in HER2 or NRG expressing tumor cells. HER3, a member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases, is frequently overexpressed in tumors; it has no active kinase domain but is activated through heterodimerization with other members of the EGFR receptor family, such as HER2.