Competition binding assays demonstrated that trastuzumab, but not MAb 2009, competed with 125I-trastuzumab binding on SK-BR-3 breast cancer cells, while MAb 2009, but not trastuzumab, competed with 125I-MAb 2009 binding on neu-expressing DHFR-G8 cells. The gene discussed is ERBB2; the disease is breast carcinoma.