We focused on the luminal-like MMTV-PyMT (PyMT) and MMTV-Her2 (Her2) genetically engineered mouse models (GEMMs) of metastatic breast cancer, which model the PI3K activation or the HER2 amplification seen in 42% or 20% of breast cancer patients, respectively [12,21]. The gene discussed is ERBB2; the disease is breast cancer.