We used a transgenic mouse model of HER2-overexpressing breast cancer, MMTV-NIC, which enforces mammary-specific expression of oncogenic Neu, the rat HER2 homolog, from a bi-cistronic transgene harboring an internal ribosomal entry site (IRES) and a Cre recombinase expression cassette, resulting in formation of metastatic mammary tumors [20]. Here, ERBB2 is linked to breast cancer.