While maintaining the HER2-binding properties and direct anti-proliferative activity of trastuzumab against sensitive cell lines, these enhanced binding properties confer additional improvements in terms of enhanced anti-tumor activity against HER2-expressing tumor cell lines in vitro; the greatest improvement was observed in ADCC activity against the lower (1+ and 2+) HER2-expressing cell lines and/or with effector cells isolated from human donors homozygous or heterozygous for the low-binding allele (158F) of CD16A. The gene discussed is FCGR3A; the disease is neoplasm.