A recombinant humanized monoclonal antibody directed against the human epidermal growth factor receptor 3 (HER3; ERBB3), with potential antineoplastic activity. Upon administration, anti-HER3 monoclonal antibody SIBP-03 targets and binds to HER3 and inhibits its activation. This may prevent HER3-mediated signaling and inhibit HER3-dependent tumor cell proliferation and differentiation. HER3, a member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases, is frequently overexpressed in tumors and is associated with poor prognosis and drug resistance; it has no active kinase domain but is activated through heterodimerization with other members of the EGFR receptor family, such as HER2.