The tissue-specific responsiveness to these hormones is partially regulated by the tissue expression of receptors that bind them.1 Ki67 is a cell proliferation marker, as it is present only during active phases of the cell cycle.2 Although expression of estrogen receptor α (ER), progesterone receptor (PR), and Ki67 serve as predictive and prognostic factors in breast cancer, little is known about their roles in normal breast tissue. This evidence concerns the gene ESR1 and breast cancer.