ERBB2 and breast cancer: In response, numerous mouse models of breast cancer have been created, characterized by a wide variety of genetic drivers including constitutive overexpression of endogenous oncogenes specifically in the mouse mammary gland (MMTV-Neu, MMTV-Cyclin D1, MMTV-Akt1 [14]), conditional overexpression of oncogenes in the mammary gland (MMTV-rtTA/TetO-NeuNT [15], MTB/TWNT [16]), nonfunctional or conditional loss of tumor-suppressor genes (Stat1−/−, MMTV-Cre/BRCA1fl/fl) [14], or overexpression of exogenous transforming oncoproteins (MMTV-PyMT [17]).