ESR1 and breast cancer: This study is the first to explore the transcriptomic profile and tumor microenvironmental differences at baseline of patients with ER+/HER2− de novo metastatic luminal breast cancer compared to patients with early (non-metastatic) luminal breast cancer to determine which aspects of the tumor microenvironment of de novo luminal breast cancer are altered and obtain perspective of the molecular mechanisms underlying breast cancer metastasis.