The space agency studied the DNA of twin astronauts Scott and Mark Kelly.
READ MORE: Astronaut twins Scott, Mark Kelly give clues to health effects of spaceflight
Scott spent a year on the International Space Station while Mark remained on Earth.
Now two years later, NASA says 7 percent of Scott's DNA has changed, meaning the brothers are no longer identical.
Read more from the NASA study: https://www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings