The erbB2 (also known as HER2 or neu) belongs to the epidermal growth factor receptor (EGFR) family. ErbB2 is a receptor tyrosine kinase with intrinsic tyrosine kinase activity. The mammalian EGFR family comprises four receptors (EGFR, ErbB2, ErbB3, and ErbB4).. The erbB2 gene is amplified or overexpressed in approximately 30% of human breast cancers. Patients with ErbB2-overexpressing breast cancer have lower overall survival rates and overexpression of ErbB2 leads to increased breast cancer metastasis. Thus, understanding the effect and mechanisms of ErbB2 signaling is critical to the success of treatment of breast cancers.