There are 98,000 human endogenous retroviral (HERV) sequences in the human genome, most inactivated by mutations. A new study in the March 22 PNAS reveals that this large number of insertions is most likely the result of germline reinfection rather than retrotransposition or complementation.

“What we think is really happening is that in the past 30 million years, there have been very many of these elements, not fixed but just present in a small number of individuals,” said Robert Belshaw, from the Department of Biological Sciences, Imperial College London, and lead author of the study.

These would have been infecting and moving about, possibly between individuals, but certainly within individuals, from somatic cells back into germline cells. Periodically, one would pick up a mutation that would stop it from moving about. “Once dead, there would be no selection against it from the host. They're just a bit of junk,...

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