Retrotransposons.

June 27, 1970 was a significant day for our understanding of both the flow of information in biological systems and the evolution of eukaryotic genomes as this was the day that Nature published back-to-back papers reporting the discovery of an enzyme that copies RNA into DNA. This soon became known as reverse transcriptase and the RNA tumour viruses in which it was detected were renamed retroviruses. The realisation that retroviruses can convert their genomic RNA into DNA provided a route by which they could integrate into the chromosomes of infected cells as Howard Temin and his colleagues had proposed some years earlier. At the time it was thought that the ability to copy RNA into DNA would be confined to retroviruses. One of the more startling outcomes of whole genome DNA sequencing has been the discovery that eukaryotes can have more reverse transcriptase genes than genes coding for any other protein, and that the largest single component of many eukaryotic genomes has been generated by reverse transcription.