After spending 342 consecutive days onboard the International Space Station from 2015 to 2016, astronaut Scott Kelly now holds the record for longest single space mission by an American. But his "One-Year" study with NASA was about more than breaking records: Its purpose was to show how prolonged time in orbit would effect Kelly's genetic makeup compared to that of his identical twin brother on Earth. Now, following recent evaluations of the two men, it appears that Scott Kelly's gene expression was significantly altered by his time in space, Space.com reports.

NASA announced the most recent findings from its Twins Study ahead of a more comprehensive paper combining the work of multiple teams of researchers that is slated for later in 2018. Like his brother Scott, Mark is also an astronaut, making the pair the only twin astronauts in history. So when NASA was looking for a way to study the long-term effects of space life, the siblings were a perfect fit.

As Scott was sending tweets and blowing bubbles on the ISS, Mark stayed on Earth to serve as the control. Biological samples taken from both subjects before, during, and after the space flight showed some dramatic differences. According to an investigation conducted by Susan Bailey of Colorado State University, Scott's telomeres, the protective "cap" at the ends of chromosomes that shorten as we age, got longer in space. The telomeres began shrinking back to preflight levels, however, a few days after Scott's return to Earth. Scott was subjected to regular exercise and a restricted diet aboard the ISS, so the new lifestyle may explain the sudden telomere boost.

Other genetic differences stuck around even months after landing. "Although 93 percent of genes' expression returned to normal post-flight, a subset of several hundred 'space genes' were still disrupted after return to Earth," acccording to a NASA press release. About 7 percent of Scott's genes may show longer-term changes, included the genes associated with DNA repair, immune health, bone formation, hypoxia (an oxygen deficiency in the tissues) and hypercapnia (excessive carbon dioxide in the bloodstream).

A long list of factors, like radiation, caloric restriction, and zero gravity, may have contributed to the results. NASA plans to use these findings to develop countermeasures against these effects, which will be essential if the agency plans to send humans to Mars, a journey that could take three times as long as Scott Kelly's ISS mission.

[h/t Space.com]

Editor's note: We updated the headline and one line of this story to more accurately reflect the research findings. We apologize for the error.