'Jumping genes' or retrotransposons are autonomous genetic elements that account for large swathes of the genomes of humans and other animals. Two papers in the 20 May 2004 issue of Nature (Vol. 429, No. 6989, pp. 268-274 and pp. 314-318) show how they might 'fine tune' the genome by modulating the expression of bona fide genes, and also how they might be harnessed to make useful additions to the biotechnological toolkit.
In one report, Jeffrey S. Han, Suzanne T. Szak and Jef D. Boeke look at LINE-1 (L1) elements, which comprise 17% of human DNA. Despite their ubiquity, there is something about the L1 sequence that makes it hard for the cellular gene-reading machinery to copy fully, so the elements jump far less efficiently than they otherwise might. This curious property also dampens gene expression in any region of the genome in which L1 elements are found. Because of this, Han and colleagues suggest that the distribution of L1 elements could modulate the activities of entire genomes. They may look like genomic junk, but L1 elements could have had a profound effect on the course of evolution.
In an accompanying report, Han and Boeke show how the difficulties in reading L1 sequences can be subverted by making perkier synthetic versions. These synthetic jumping genes copy and insert themselves throughout the genome 200 times as effectively as the natural versions, a property that could make them useful in the manipulation of genomes.
Frederic Bushman discusses the research in an accompanying News and Views article.
Jef D. Boeke (The Johns Hopkins University School of Medicine, Baltimore, MD, USA) paper no:  and 
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Frederic Bushman University of Pennsylvania, Philadelphia, PA, USA)
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