We have reported that human breast cancer cells fail to express, or express at very low levels, the circadian clock gene Per2 and, upon re-expression of Per2, MCF-7 cell growth was inhibited and apoptosis induced.Our aim in this study was to investigate the difference in the expression of clock and clock controlled genes between MCF-10A human mammary epithelial cells and MCF-7 breast cancer cells and to determine how the pineal hormone, melatonin, and the retinoid-related orphan receptor alpha-one (RORα1) influences clock gene expression in both normal and malignant breast epithelial cells.