ERBB2 and breast cancer: Therefore, we used two preclinical mouse models of breast cancer: MMTV-Neu, which replicates HER2-positive breast cancer and MMTV-PyMT transgenic mice, an aggressive autochthonous model of triple negative breast cancer (mice do not express estrogen, progesterone, or HER2 receptors).27–29 In MMTV-Neu mice, unactivated Neu (ErbB2/HER2) protein expression is targeted to the mammary gland by a MMTV promoter.27 ErbB2/HER2 is a tyrosine kinase receptor that promotes tumor growth, and this protein is overexpressed in 20–30% of breast cancer patients.