Effects of retroviruses on host genome function.

For millions of years, retroviral infections have challenged vertebrates, occasionally leading to germline integration and inheritance as ERVs, genetic parasites whose remnants today constitute some 7% to 8% of the human genome. Although they have had significant evolutionary side effects, it is useful to view ERVs as fossil representatives of retroviruses extant at the time of their insertion into the germline and not as direct players in the evolutionary process itself. Expression of particular ERVs is associated with several positive physiological functions as well as certain diseases, although their roles in human disease as etiological agents, possible contributing factors, or disease markers-well demonstrated in animal models-remain to be established. Here we discuss ERV contributions to host genome structure and function, including their ability to mediate recombination, and physiological effects on the host transcriptome resulting from their integration, expression, and other events.