GENES MAKE UP ONLY ABOUT 2 PERCENT of the human genome. The rest was for many years ignored as “junk DNA.” But over the past decade biologists have come to understand that this space is an incredibly important part of the genetic code, home to a vast unexamined treasure trove of information that controls how genes behave. A more thorough investigation of junk DNA could upend our understanding of the delicate interplay between genes and the environment and could lead to entirely new strategies in medicine's endless struggle against disease.