An immunoglobulin (Ig) G1 monoclonal antibody directed against both human epidermal growth factor receptor 3 (HER3 or ERBB3) and human epidermal growth factor receptor (EGFR), with potential antineoplastic activity. Anti-EGFR/HER3 Monoclonal Antibody MEHD7945A binds to both EGFR and HER3 and inhibits their activation. This may prevent EGFR/HER3-mediated signaling and inhibit EGFR/HER3-dependent tumor cell proliferation. In addition, MEHD7945A induces antibody-dependent cell-mediated cytotoxicity (ADCC) against EGR/HER3-expressing tumor cells. EGFR and HER3, members of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases, are frequently overexpressed in tumors; their expression is associated with both poor prognosis and drug resistance.