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). The gene discussed is ERBB2; the disease is breast carcinoma.