When the scFv of trastuzumab was fused to either the N- or C-terminus of the heavy chain of the IgG scaffold of 609A, it showed a significantly reduced binding affinity for BT474 cells (a HER2-overexpressing breast cancer cell line), and was much less potent in inhibiting proliferation of the tumor cells, compared to trastuzumab (data not shown). The gene discussed is ERBB2; the disease is breast carcinoma.