Spending a year in space not only changes your outlook, it transforms your genes.

Preliminary results from NASA’s Twins Study reveal that 7% of astronaut Scott Kelly’s genes did not return to normal after his return to Earth two years ago.

The study looks at what happened to Kelly before, during and after he spent one year aboard the International Space Station through an extensive comparison with his identical twin, Mark, who remained on Earth.

NASA has learned that the formerly identical twins are no longer genetically the same.

The transformation of 7% of Scott’s DNA suggests longer-term changes in genes related to at least five biological pathways and functions.