ERBB3 and neoplasm: Monovalent binding to the trigger molecule CD3 on T-cells and bivalent binding to the membrane-proximal tumor antigen HER3 enabled effective formation of immune synapses and triggering T-cell mediated tumor cell lysis, emphasizing the suitability of HER3 as a tumor target for redirecting T-cells to tumor cells discriminating between moderate and low HER3-expressing target cells.