Taken together, our data indicate that the anti-erbB3 Ab MM-121 exhibits the potential to overcome paclitaxel resistance and significantly enhances paclitaxel-mediated anti-proliferative/anti-survival effects in erbB2-overexpressing breast cancer cell lines, with medium and high erbB3 expression, associated with its inhibition of the erbB3/PI-3 K/Akt signaling. This evidence concerns the gene AKT1 and breast cancer.