ERBB2 and neoplasm: In metastatic breast cancer models that are largely driven by either PyMT (polyomavirus middle T antigen) or MMTV‐Neu (Erbb2) oncogenes, gene expression profiles of primary tumor have mostly ‘luminal‐like’ or epithelial gene expression signature with only rare cells with mesenchymal phenotype (Herschkowitz et al., 2007).