Cells can stop dividing and remain in a nonproliferative state (like most differentiated cells in an organism), but under some conditions the cells can reenter the cell division cycle and begin to proliferate again. Sang et al. provide evidence that a transcriptional repressor known as Hairy and Enhancer of Split1 (HES1) has an important role in determining whether a cell can resume cell division. When proliferation of human fibroblast cells in culture was inhibited, reduction of Hes1 activity prevented cells from reinitiating cell division once the block to proliferation was removed. Inappropriate reentry into the cell cycle is a property of tumor cells, and treatments that inhibited Hes1 function in skeletal muscle tumor cells also inhibited proliferation and promoted differentiation. L. Sang, H. A. Coller, J. M. Roberts, Control of the reversibility of cellular quiescence by the transcriptional repressor HES1. Science 321 , 1095-1100 (2008). [Abstract] [Full Text]