When a fertilized egg develops into a fetus, for example, some of its cells develop into the placenta, an organ that draws in nutrients from the mother’s tissues. The cells in the outer layer of the placenta fuse together, sharing their DNA and other molecules. Heidmann and other researchers have found that a human endogenous retrovirus gene plays a crucial role in that fusion. The cells in the outer placenta use the gene to produce a protein on their surface, which latches them to neighboring cells. In our most intimate moment, as new human life emerges from old, viruses are essential to our
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