In any given cell, most genes are turned off. Turning genes off and keeping them off is as important as switching them on. Large sections of unneeded genes must be reliably repressed for millions of cell divisions so cells can differentiate and maintain their specialized identities. During cell division, the stability of every chromosome depends upon conserved silent sections of chromosomes near landmark structures, such as the centromeres and telomeres.
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. Gene silencing proteins spread along the chromatin fiber from an initiation site, and they bind like glue to large regions of DNA packed into nucleosome beads. Sir2 is the central player in silencing complexes in yeast. As such, it breaks two old bonds and makes a new one. At least one of these events is required for gene silencing.