Elgemtumab (development code LJM716) is an investigational, fully human immunoglobulin G (IgG) monoclonal antibody developed as a targeted therapy for cancers driven by HER3 signaling. It functions as a potent and specific HER3 antagonist. Its mechanism of action is to bind with high affinity to the extracellular domain of HER3, thereby effectively blocking both ligand-dependent and ligand-independent HER3 signaling pathways. This dual blockade prevents the activation and subsequent dimerization of HER3 with other ErbB family receptors (like HER2 or EGFR), a critical step for downstream oncogenic signaling that promotes tumor cell survival, proliferation, and resistance to therapy.