A gene is a DNA sequence that encodes the instructions for when and where to make a particular protein. But most of the DNA in our genome—well over ninety percent—is not composed of genes.
The argument over the role of this seemingly extraneous DNA has swung back and forth. In the 1970s, it was thought to be generally useless junk. But in 2012, the ENCODE consortium (the ENCyclopedia Of DNA Elements—cute, right?) posited that most of the DNA had some sort of activity. Earlier this year, a new analysis insisted again that it's just junk.
Even as that debate was raging among researchers, viruses have used some of the noncoding DNA for their own purposes: to hijack our cellular metabolism and promote their own replication. Results are reported in Science.