A new study from NASA has found that astronaut Scott Kelly’s genes are no longer identical to those of his identical twin after spending a year in space.

Preliminary results from NASA’s Twins Study found that seven percent of Kelly’s genes no longer match those of his twin, Mark. Scott Kelly spent one year aboard the International Space Station during the study, while his brother remained on Earth.

While in space, researchers monitored Kelly’s metabolites, cytokines, and proteins to learn how space travel affects biological systems.

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Though most of Kelly’s biological changes returned to baseline levels after returning to Earth, seven percent of his genes point to possible long-term changes, according to the study.

NASA's preliminary findings were validated this week, according to Space.com.

“The Twins Study has benefited NASA by providing the first application of genomics to evaluate potential risks to the human body in space,” according to a release from the agency.

Kelly’s one-year mission was twice as long as astronauts’ typical missions aboard the International Space Station, but it is a “stepping stone” to an eventual three-year mission to Mars, the agency said.

Mark Kelly, who is also a retired NASA astronaut, is the husband of former Rep. Gabrielle Giffords (D-Ariz.).