A new study has reported a mechanism of action for the observation that prolonged disruption of circadian cycles (for example, night shift work) may increase breast cancer risk. The study focused on the role of the central circadian regulator, CLOCK, in breast cancer. Significant correlations were found between variations in the CLOCK gene and breast cancer risk. Lower levels of CLOCK expression were observed in healthy controls compared to both normal and tumor tissues from breast cancer patients. In other words, the CLOCK gene was found to be overexpressed in the breast tissue of those with breast cancer. This effect was more pronounced in women with ER-/PR- tumors. The authors conclude that variants in a gene responsible for regulating the body’s circadian rhythm influence breast cancer development. Furthermore, they have identified a set of circadian gene variants that may be breast cancer susceptibility biomarkers.