UNITED NEWS INTERNATIONAL (UNI) — Going to space would be life changing.

But for American astronaut Scott Kelly, it actually changed his DNA.

Scott and his brother, Mark, are the only identical twin astronauts in history.

Scott spent nearly a year living on board the International Space Station from 2015 to 2016 while Mark stayed put on terra firma.

By comparing the brothers when Scott returned, researchers were able to see how long-term space travel directly affects the body.

NASA recently announced the results of the so-called “Twins Study.”

The space agency said some of Scott’s genes involved in bone development, oxygen deprivation, immune system responses and DNA repair were no longer identical to Mark’s.

Researchers think the stress of space triggered reactions in Scott’s immune system, which then altered certain aspects of his DNA to cope with his low gravity, low oxygen environment.  

The study is expected to help scientists prepare for future manned missions to Mars.