ERBB3 and breast cancer: The proliferation of ErbB2-overexpressing breast cancer cells requires ErbB3, indicating that the ErbB2/ErbB3 heterodimer acts as a potent oncogenic unit [6], which couples to the downstream Ras/MAPK pathway through a single ErbB2 C-terminal phosphorylation site [7], and to the PI3K/Akt pathway through six ErbB3 C-terminal phosphorylation sites [8].