ERBB3 and breast cancer: To study whether inactivation of erbB3 signaling with MM-121 may overcome paclitaxel resistance and facilitate paclitaxel-mediated inhibitory activity against erbB2-overexpressing breast cancer, the SKBR3.B3.1 and SKBR3.B3.2 cells that show resistance to paclitaxel due to ectopic expression of erbB3 in SKBR3 cells [27] were used to investigate inhibitory effects of MM-121 on erbB3 signaling and enhancement of paclitaxel-mediated anti-proliferative/anti-survival effects.