A human monoclonal antibody directed against the human epidermal growth factor receptor ErbB3 (Her3) with potential antineoplastic activity. Upon administration, anti-ErbB3 monoclonal antibody REGN1400 binds to ErbB3 and prevents neuregulin 1 ligand binding to ErbB3, which may result in an inhibition of ErbB3-dependent phosphatidylinositol-3 kinase (PI3K)/Akt signaling. This eventually leads to the inhibition of cellular proliferation and differentiation. ErbB3, a member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases, is frequently overexpressed in a variety of solid tumors and its overexpression generally correlates with poor prognosis and tumor resistance.