OUR RETROVIRAL HERITAGE

Darwin could not have foretold that we are descended from viruses as w ell as from apes. While there is clear evidence that viral diseases, s uch as polio and rabies, affected ancient civilizations, viruses were not defined until the early years of this century, shortly after the r ediscovery of mendelian genetics. That retroviral genomes can oscillat e between infectious and genetic modes of transmission seemed preposte rous before the discovery of reverse transcription in 1970. Those of u s who had earlier provided mendelian evidence for germ-line transmissi on of retroviruses were subject to friendly ridicule. Today, the shunt ing of genetic elements between chromosomes and RNA, and the generatio n of processed pseudogenes, seems commonplace. It is timely, however, to revisit the topic of human endogenous retroviruses-the subject of t his article.