A monoclonal antibody directed against the human epidermal growth factor receptor ErbB3 (Her3) with potential antineoplastic activity. Anti-ErbB3 monoclonal antibody KTN3379 binds to and prevents ligand binding to ErbB3, which may inhibit ErbB3-dependent phosphatidylinositol-3 kinase (PI3K)/Akt signaling and may lead 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.