Since miR-200b-3p and miR-200b-5p were both expressed in our MDA-MB-231-miR-200b cell line and expression of both -3p and -5p were observed at physiologically relevant and similar endogenous levels in ER+ breast cancer cell lines, we next sought to evaluate the preference for isomiR expression for both of these miRNAs in our deep sequencing analysis. This evidence concerns the gene ESR1 and breast cancer.