In the present study, we investigated the effect ofcombined treatment with a humanized anti-HER2 monoclonal antibody, rhumAbHER2 (trastuzumab), and an anti-oestrogen, ICI 182,780, onthe cell growth of three human breast cancer cell lines which respectively express different levels of ER and HER2. The gene discussed is ERBB2; the disease is breast carcinoma.