Human epidermal growth factor receptor 2 (HER2/ERBB2) is one of the most intensively investigated oncogenes in breast cancer research. Its amplification or over-expression defines a biologically aggressive subtype of breast cancer, historically associated with poor prognosis. Estrogen receptor (ER) signaling represents another dominant axis of breast carcinogenesis, governing tumor growth, differentiation, and response to endocrine therapy. Accumulating evidence demonstrates that ER and HER2 pat