ERBB2 and breast cancer: Human epidermal growth factor receptor 2 (HER2/ERBB2) is overexpressed in approximately 25% of breast cancers and drives hyperactivation of the HER2 pathway via downstream signaling initiated by receptor homo- or hetero-dimerization with other HER family members (HER2/HER2, HER2/EGFR, HER2/HER3).