ERBB2 and breast carcinoma: MMTV-Neu mice respond to trastuzumab, confirming the relevance of the model for studying ErB2/HER2-positive breast cancer.27 Expression of the polyoma middle T oncoprotein (PyMT) in the mammary epithelium drives the formation of aggressive tumors in the mammary gland of mice.28 These highly aggressive tumors do not express progesterone, estrogen, or HER2 receptors and are unresponsive to conventional therapies, mirroring human TNBC.30