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. The gene discussed is MKI67; the disease is breast cancer.