A human monoclonal antibody directed against the human epidermal growth factor receptor HER3 (ErbB3) with potential antineoplastic activity. Elgemtumab 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.