Scientists say this "junk" DNA is found in regions of the genome dating back hundreds of thousands of years, The New York Times reported Thursday.
Some of those genes, geneticists found, can rise from the dead like zombies to cause facioscapulohumeral muscular dystrophy, or FSHD.
This is the first time, they say, they have observed a dead gene come back to life and cause a disease.
FSHD affects about one in 20,000 people, causing a progressive weakening of muscles in the upper arms, the shoulders and in the face.
The suspect genes had seemed permanently disabled, but "the first law of the genome is that anything that can go wrong will," said Dr. Francis Collins, a human geneticist and director of the National Institutes of Health.
About 20 years ago, geneticists zeroed in on the region of the genome that seemed to be involved in FSHD: a long chain of repeated copies of a dead gene.
The dead gene was also repeated in another area of the genome, but that area of repeats seemed innocuous, unrelated to the disease.
Under certain circumstances, geneticists have discovered, the "innocuous" genes can come back to life, triggering FSHD.
The findings suggest a way to search for treatments, says
David Housman, a geneticist at M.I.T. who was not involved in the research but is chairman of the scientific advisory board of the FSHD Society, an advocacy group.
"It has made it clear what the target is," he said. "Turning off that dead gene. I am certain you can hit it."