ESR1 and breast cancer: Estrogen (E2) and the estrogen receptor (ER) have been shown to modulate miRNAs such as miR-125a[6] and miR-429[7] in mouse uterus, miRNA-20a and miRNA-21 in normal endometrial glandular epithelial cells[8], Let-7 family, miR-27a, miR320 and miR-424[9] in EC cells, miR-104[10], miR-7[11], miR-21[12], miR-30c and miR-103[13] in breast cancer cells, miR-135b in colorectal cells[14] and miR-203 in vascular smooth muscle cells[15].