A time to divide: does the circadian clock control cell cycle?

A variety of molecular, genomic and epidemiological evidence has linked cell cycle regulation to circadian rhythms. In a recent issue of Molecular Cell, Koeffler and colleagues show that PER1 sensitizes human cancer cells to ionizing radiation-induced apoptosis. In addition, PER1 expression was found to be downregulated in human tumors, suggesting a tumor suppressor role for the PER1 protein. The significance of PERs, clocks, and cell cycle control is discussed.