Astronaut Twin Study Shows Possible Stress of Long Space Missions

Early results of an unprecedented study to determine the effects of space travel show genetic differences between astronaut Scott Kelly, who spent a year in space, and his identical twin brother who stayed on Earth.

There were changes in Kelly’s gene expression and DNA methylation, meaning the genes were essentially chemically activated, according to preliminary results published Thursday on the journal Nature’s website.

“Almost everyone is reporting that we see differences,” said Christopher Mason, a geneticist at New York’s Weill Cornell Medical College, according to Nature.

It’s unclear what the preliminary findings indicate. The brothers will be closely followed for at least four more years.

Kelly and his brother Mark Kelly, who served as the test’s control, agreed to help scientists learn more about the effects of space on one’s body. The latest results were presented at the annual meeting of the Human Research Program in Galveston

Kelly spent 340 days in space before returning home in March 2016. TIME covered the Kelly brothers’ mission extensively in an Emmy-nominated 12-episode series, which can be viewed at time.com/space.