Overexpression of ERBB2 (also known as HER2/NEU) is associated with human breast cancers, and mouse models of HER2+ tumors, including MMTV-NIC transgenic mice (expressing an activated oncogenic human ERBB2), have provided many insights into how ERBB2 induces tumors and promotes metastases [18–20]. This evidence concerns the gene ERBB2 and breast cancer.