Alu elements are the most abundant family of non-LTR TEs, namely SINEs, in the human genome. They have been in the human genome for over 60 million years + are still active (transposition-competent). Alus occur around a million times in the human genome.
Alus are usually 300bp in length + their structure is a dimer of two sequences evolved from the small cytoplasmic 7SL RNA gene, with an A-rich spacer in between + terminated by an oligomer of T's.
Alus rely on the retrotransposition machinery of group L1 LINEs + their insertions have been associated with many diseases, such as breast cancer and diabetes.
They come in three flavours: AluJ, AluS and AluY, which are listed in chronological order of appearance in the genome. The AluJ family is hypothesised to be completely inactive in the human genome; a small proportion of AluS elements are still active; and the AluY group is completely active.
1. Cordaux, R. and Batzer, M.A., 2009. The impact of retrotransposons on human genome evolution. Nature reviews. Genetics, 10(10), p.691.
3. Bennett, E.A., Keller, H., Mills, R.E., Schmidt, S., Moran, J.V., Weichenrieder, O. and Devine, S.E., 2008. Active Alu retrotransposons in the human genome. Genome research, 18(12), pp.1875-1883.