Cited research: Genome Res. doi:10.1101/gr.106419.110 (2010)

Researchers have mapped the genomic locations of almost every member of a family of human retrotransposons — short DNA segments thought to make up as much as one-third of the genome. These elements — which can affect physical traits — copy and then paste themselves back into the genome at various locations. Despite their abundance, they are not as well studied as other forms of genomic variation.

Adam Ewing and Haig Kazazian at the University of Pennsylvania in Philadelphia used high-throughput sequencing to analyse the genomes of 25 individuals, including 15 people who were unrelated. They identified 1,139 insertion sites for the L1 retrotransposon family, of which 247 were previously unknown. They estimate that there are 3,000–10,000 relatively common L1 elements in the human population and that the rate at which a human genome gains a new L1 insertion is 1 in 140 births per generation. C.L.