Controlled in part by telomere length. Telomeres cap chromosomes and shorten with age of the cell, limiting gene expression control:-"Specifically, the team showed that when a telomere is long, the endcap can form a loop with the chromosome that brings the telomere close to genes previously considered too far away to be regulated by telomere length. Once the telomere and the distant genes on the same chromosome are close to each other, the telomere can generally switch those genes off.-"Conversely, when telomeres are short, the chromosome does not form a loop and the telomere can no longer influence whether target genes are switched on or off.-"The researchers were able to identify three genes whose expression patterns are altered by telomere length but believe this number is the just the tip of the iceberg.-"'We have developed the concept that telomere shortening could be used as a timing mechanism to respond to physiological changes in very long-lived organisms, such as humans, to optimize fitness in an age-appropriate fashion," said Dr. Wright.'"-http://medicalxpress.com/news/2014-11-scientists-uncover-mechanism-cells-impacting.html