French researchers have resurrected a retrovirus that became trapped in the human genome about five million years ago. Pieced together from existing sequences in human DNA, the reconstructed virus was able to infect mammalian cells weakly, suggesting that it works similarly to the extinct organism.
Image: © CSHL PRESS 2006
FROM THE ASHES: Researchers have reconstructed an extinct retrovirus and shown it (red) can infect human cells (blue).
Retroviruses insert their DNA into a host genome in order to reproduce, but if they stick around long enough they might undergo a mutation that keeps them from popping back out. Nearly 8 percent of the human genome consists of such captured retroviral DNA sequences, which gradually become garbled over the millennia. A few of those acquired more recently, however, have nearly complete sequences. They belong to an extinct family of retroviruses called HERV-K (for human endogenous retrovirus, K type). Some of these HERV-K elements seem to play a role in placental development and even cause viruslike particles to form in certain tumors. Researchers could not isolate a functioning, infectious HERV-K virus from human samples to study its possible function, though.
Read rest of the story at Scientific American
Biosingularity 
Nice to know.