ESR1 and breast cancer: To address the question of whether primary and metastatic ER+ breast cancer samples are generally similar in terms of biomarker prevalence, and hence whether primary tissue is an accurate indicator of biomarker status in later stage patients, we evaluated a panel of asynchronously collected matched primary and metastatic tumors with a panel of biomarkers related to proliferation, epithelial-mesenchymal biology and PI3K pathway signaling.