The space agency studied the DNA of twin astronauts Scott and Mark Kelly.
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 genes changed, showing how the human body adapts to its environment.
Read more from the NASA study: https://www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings
CORRECTION: A previous version of this story said the brothers were no longer identical. NASA has since issued a clarification saying, "Mark and Scott Kelly are still identical twins; Scott's DNA did not fundamentally change. What researchers did observe are changes in gene expression, which is how your body reacts to your environment."