Recent studies indicate that greater than 10% of the human and mouse genome appears to consist of integrated DNA copies of RNA molecules. These sequences include retroviruses, retrovirus-like DNAs, retrotransposons, and retrotranscripts and represent more than 500,000 separate integration events. The nature of the enzymes used for the reverse transcription from RNA to DNA and for integration of the DNA copies into chromosomal DNA is unknown. A major evolutionary effect of these integrations would have been mutation. Thus, present-day organisms are those that survived this mutational load.