Transposable genetic elements are DNA segments that move around in the genome. Many biologists consider them to be a form of "selfish DNA," a kind of genetic parasite that serves no useful function for the host, but remains in the genome because it is efficient at getting itself copied. They can be present in large numbers of copies. In humans, more than a million copies of a single element, called Alu, account for about ten percent of the entire genome.
Insertion of a transposable element can disrupt a gene, creating a pseudogene. When a transposable element moves, it occasionally also takes a gene with it, placing it in a new position under the control of different regulatory elements. Alternatively, it may move an enhancer, thus affecting both the gene whose enhancer was removed and the gene (if any) it is now placed closer to. Some transposable elements themselves contain enhancers, further increasing the chances of altering gene expression when they are inserted in a new location.
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