We found that, while these HER2-specific mAbs demonstrated a synergistic effect against the growth of BT-474 cells in vitro, consistent with a previous report (14), this synergy was not seen in HER2+/HER3+ BC models such as KPL-4 and Au565 cells, or in HER2+/HER3– models, such as SKOV3 (31) or NIH/3T3-expressing HER2 in vitro (Figure 1A). This evidence concerns the gene ERBB3 and breast cancer.