Finally, M/D-driven mammary carcinomas not only fail to express erb-b2 receptor tyrosine kinase 2 (ERBB2, best known as HER2), but most often also preserve estrogen receptor 1 (ESR1) and progesterone receptor (PGR) expression throughout the oncogenic process [13], hence exhibiting fundamental differences to other mouse models of breast cancer expressing HRs such as MMTV-PyMT mice. This evidence concerns the gene ESR1 and breast cancer.