Activation and overexpression of epidermal growth factor receptor (EGFR, also known as ErbB) family members, including EGFR (ErbB1 or HER1), HER3 (ErbB3), HER4 (ErbB4), and HER2 (ErbB2), govern multiple important cellular processes in breast cancer. The gene discussed is ERBB4; the disease is breast cancer.