NASA astronaut Scott Kelly spent nearly a year in space so the agency could study the long-term effects on the human body, in preparation for longer missions that could one day take us to Mars and beyond. Upon his return, he discovered his overall health was not to be what it used to be — and NASA had a Kelly replica at home to compare Scott against. That’s his twin brother, Mark, an astronaut who stayed on Earth while his brother was circling our planet.

The early conclusions of a recent NASA study implied that Scott’s DNA has been altered by 7%, which seemed to be a startling discovery of what space can do to the human body. However, that’s a wrong interpretation of NASA’s announcements. It turns out that while Scott’s DNA was affected by exposure to space, his DNA is still nearly identical to his brother’s, as it should be.

What NASA really discovered is that, while DNA mutations were observed after Scott’s return from space, it was his gene expression that changed by 7%, compared to his brother. However, the agency’s choice of words may have made it sound like Scott’s DNA was permanently altered. The agency updated that press release to make it clear that Scott and Mark are still identical twins.

The DNA code that serves as the programming language for any living body, including humans, has not changed for Scott. Gene expression did change by 7%, though. But that only means Scott’s body adapted the way it functions after his exposure to space, even though it’s been taking information from the same DNA code.

Scott’s 7% gene expression differences concern the immune system, DNA repair, bone formation, and more, according to the NASA. The phenomenon happens on Earth as well, and it’s a response to a person’s environment.

“To have 7 percent of his gene expression changed after the spaceflight does not mean that 7 percent of the DNA changed, or that those changes were necessarily due to mutations,” UC Davis geneticist Nichole Holm told The Verge — she did not work on the Twins Study.

“The two are still very much identical twins,” Holm said. “They possessed different mutations before and after the flight, and Scott experienced different changes in his RNA, not DNA. But their DNA is still nearly identical and much more similar to each other than to any other person on earth (or in space).”

NASA also confirmed that Scott’s DNA did not change.

Scott’s DNA did not fundamentally change,” a NASA spokesperson said. “What researchers did observe are changes in gene expression, which is how your body reacts to your environment. This likely is within the range for humans under stress, such as mountain climbing or SCUBA diving.”

The spokesperson continued, “We are at the beginning of our understanding of how space flight affects the molecular level of the human body. NASA and the other researchers collaborating on these studies expect to announce more comprehensive results on the Twins Studies this summer.”

NASA will announce more details soon, hopefully in clearer wording that will avoid confusion.

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