To explore whether the anti-erbB3 Ab MM-121 may enhance the activity of trastuzumab against erbB2+ breast cancers, we investigated the combinatorial effects of MM-121 and trastuzumab on erbB3 signaling and cell proliferation in two erbB2+ breast cancer cell lines (SKBR3 and BT474). This evidence concerns the gene ERBB3 and breast cancer.