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Journal of Biomedicine and Biotechnology
Volume 2006 (2006), Article ID 75327, 5 pages
Mini-Review Article

The Genomic Distribution of L1 Elements: The Role of Insertion Bias and Natural Selection

1Department of Biology, Queens College, City University of New York, Flushing 11367, NY, USA
2Graduate School and University Center, City University of New York, New York 10016, NY, USA

Received 5 March 2005; Revised 6 December 2005; Accepted 13 December 2005

Copyright © 2006 Todd Graham and Stephane Boissinot. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


LINE-1 (L1) retrotransposons constitute the most successful family of retroelements in mammals and account for as much as 20% of mammalian DNA. L1 elements can be found in all genomic regions but they are far more abundant in AT-rich, gene-poor, and low-recombining regions of the genome. In addition, the sex chromosomes and some genes seem disproportionately enriched in L1 elements. Insertion bias and selective processes can both account for this biased distribution of L1 elements. L1 elements do not appear to insert randomly in the genome and this insertion bias can at least partially explain the genomic distribution of L1. The contrasted distribution of L1 and Alu elements suggests that postinsertional processes play a major role in shaping L1 distribution. The most likely mechanism is the loss of recently integrated L1 elements that are deleterious (negative selection) either because of disruption of gene function or their ability to mediate ectopic recombination. By comparison, the retention of L1 elements because of some positive effect is limited to a small fraction of the genome. Understanding the respective importance of insertion bias and selection will require a better knowledge of insertion mechanisms and the dynamics of L1 inserts in populations.