The human genome contains billions of pieces of information and around 22000 genes, but not all of it is, strictly speaking, human. Eight percent of our DNA consists of remnants of ancient viruses, and another 40 percent is made up of repetitive strings of genetic letters that is also thought to have a viral origin. Those extensive viral regions are much more than evolutionary relics: They may be deeply involved with a wide range of diseases including multiple sclerosis, hemophilia, and amyotrophic lateral sclerosis (ALS), along with certain types of dementia and cancer.

For many years, biologists had little understanding of how that connection worked—so little that they came to refer to the viral part of our DNA as dark matter within the genome. “They just meant they didn’t know what it was or what it did,” explains Molly Gale Hammell, an associate professor at Cold Spring Harbor Laboratory. It became evident that the virus-related sections of the genetic code do not participate in the normal construction and regulation of the body. But in that case, how do they contribute to disease?

Eight percent of our DNA consists of remnants of ancient viruses, and another 40 percent is made up of repetitive strings of genetic letters that is also thought to have a viral origin.