So cells can differentiate and maintain their specialized identities, large sections of unneeded genes must be turned off. During cell division, the stability of every chromosome depends upon sections of chromosomes remaining silent. Cancer is believed to be due, in part, to the loss of cell identity and unstable chromosomes. Aging also may be linked to silencing activities. Only recently have the detailed biochemical mechanisms behind this process of turning genes on and off started to come to light. That is thanks to the work of several labs on the best defined model system, the budding yeast, and Sir2, a repressor protein that is required for all known examples of yeast silencing.