ESR1 and neoplasm: Here we present that the expression of cell-free miRNAs correlates well with their intracellular counterparts, which suggests a much broader application of cell-free miRNAs: (i) they might be used as extracellular markers in cellular studies to monitor cellular physiology and/or target mRNA expression; (ii) they may be used in the development of cellular assays in biotechnology; (iii) their application in liquid biopsy as diagnostic biomarkers might be extended to providing information about the pathophysiology of tumor cells (e.g., ERα expression and estrogen/tamoxifen sensitivity).