ESR1 and cancer: Although previous studies were smaller in size, the distributions of marker expression we observed in the current study are comparable to those from previous studies of normal breast tissue.25–30 In study populations where the majority or all women were premenopausal, investigators have observed 3–7% ER-positive cells25–27,29,30, 12–29% PR-positive cells,25–27 and 3% Ki67-positive cells,30 in normal breast epithelium of cancer-free women.