Here is the version most people walked away with in the spring of 2018: a year in space rewrote an astronaut’s DNA so completely that he came home no longer an identical twin to his own brother. It is a great story. It is also wrong, and the people who ran the study spent months saying so.
We are not geneticists or clinicians, and nothing here is medical advice. This is our reading of a widely misreported study and the correction that followed. The Twins Study looked at one pair of brothers in great detail, so its findings describe what happened to two people, not a rule about everyone.
The astronaut is Scott Kelly. His identical twin, Mark Kelly, stayed on the ground as a comparison. NASA’s Twins Study collected more than 300 samples over 27 months, before, during and after Scott’s mission, to see how a long stretch in orbit changes the body. That mission ran 340 days, the longest single spaceflight by a NASA astronaut at the time.
Gene expression is not the same as your DNA
The whole misunderstanding turns on one distinction. Your DNA is the fixed instruction manual you were born with. Gene expression is which of those instructions your body is actively using at a given moment, and how strongly. NASA described what its researchers actually saw as “changes in gene expression, which is how your body reacts to your environment.” That is NASA’s own plain phrasing, not a textbook definition, but it captures the point. Expression shifts all the time in response to stress, illness, altitude, exercise. The underlying code does not.
Susan Bailey, a professor at Colorado State University who worked on the study, drew the line bluntly. She told Inside Edition that the finding was “not 7 percent of his genes or the DNA, but a very different thing, and that is the genes themselves that are expressed.” Same seven percent, completely different meaning.
The real number, and why NASA called it minimal
Most of the gene-expression changes that showed up during spaceflight went back to normal once Scott was home. By NASA’s account, the lingering portion was “only 7 percent of the gene expression that changed during spaceflight that had not returned to preflight after six months on Earth.” The other roughly 93 percent had returned to normal within about six months of landing.
The genes still behaving differently were not random. They clustered around things you would expect a body to notice after a year in orbit: the immune system, DNA repair, bone formation, and the body’s response to low oxygen and high carbon dioxide. NASA’s own read on the scale was flat: “This change of gene expression is very minimal,” the agency wrote, adding that it “likely is within the range for humans under stress, such as mountain climbing or SCUBA diving.” Keep in mind that last line is an estimate of comparable strain, not a measured match.
How a modest finding went viral in the wrong shape
The distortion has a clear origin. NASA’s early summary said about seven percent of Scott’s gene expression hadn’t reset. A run of outlets compressed that into “seven percent of his DNA changed,” then took the extra step of concluding the brothers were no longer identical twins.
Two errors stacked on each other: expression became DNA, and a small, mostly reversed shift became a permanent rewrite.
Bailey’s rebuttal cut through the math. “If 7 percent of his genes changed,” she said, “not only would he not be a twin, he wouldn’t be a human.” A seven percent change to the actual genetic code would put someone outside the species, never mind outside a sibling match.
NASA reissued its release with a clarifying note attached. The correction was about as plain as an agency statement gets: “Mark and Scott Kelly are still identical twins; Scott’s DNA did not fundamentally change.” Bailey seemed almost resigned to which version would stick. “The true story isn’t near as interesting as the fake news story was,” she said.