An Alu element is a short stretch of DNA originally characterized by the action of the Arthrobacter luteus restriction endonuclease. Alu elements are the most abundant transposable elements in the human genome, present in excess of one million copies. Alu elements were thought to be selfish or parasitic DNA, because their sole known function is self reproduction. However, they are likely to play a role in evolution and have been used as genetic markers. They are derived from the small cytoplasmic 7SL RNA, a component of the signal recognition particle. Alu elements are highly conserved within primate genomes and originated in the genome of an ancestor of Supraprimates.
Alu insertions have been implicated in several inherited human diseases and in various forms of cancer. From Wikipedia
