Here we provide experimental evidence indicating that MM-121 inactivates erbB3 (Figures 1B and 4A), significantly enhances paclitaxel-induced anti-proliferative/anti-survival effects (Figures 1A, 4B, 5A, and 6), and apoptosis (Figures 2B, 2C, 4C, 4D, and 6) in erbB2-overexpressing breast cancer cells with either medium (SKBR3, SKBR3.neo1, and BT474-HR20) or high erbB3 (SKBR3.B3.1 and SKBR3.B3.2) expression. Here, ERBB3 is linked to breast carcinoma.