To assess the ability of F3-IgG to inhibit the proliferation of HER2-overexpressing breast carcinoma cells, we measured the proliferation of BT474, MCF7/HER2 and MCF7 breast cancer cell lines in the presence of trastuzumab, F3-IgG, or an isotype-matched control antibody. The gene discussed is ERBB2; the disease is breast carcinoma.