Ultra-conserved element

An ultra-conserved element is a region of human DNA of length 200 nucleotides or greater, which is entirely conserved (identical nucleic acid sequence) in both rats and mice. Despite often being noncoding DNA, some ultra-conserved elements have been found to be transcriptionally active, giving non-coding RNA molecules.

Evolution
Perfect conservation of these long stretches of DNA is thought to imply evolutionary importance as these regions appear to have experienced strong negative selection for 300-400 million years. The probability of finding ultra-conserved elements by chance (under neutral evolution) has been estimated at less than 10−22 in 2.9 billion bases.

Functions
481 ultra-conserved elements have been identified in the human genome. A small number of those which are transcribed have been connected with human carcinomas and leukemias. For example, TUC338 is strongly upregulated in human hepatocellular carcinoma cells. A study comparing ultra-conserved elements between humans and Fugu rubripes proposed an importance in vertebrate development. Several ultra-conserved elements are located near transcriptional regulators or developmental genes.