“After year in space, astronaut Scott Kelly no longer has same DNA as identical twin,” the headline of a story on the Today show’s website, published Thursday, declared. Seven percent of his DNA, the story says, “has not returned to normal since he returned from space.”

Pretty amazing news, right? Too bad it’s not true.

This week, dozens of news organizations published stories with this or similar information. They cited a NASA study on the effects of space travel on the human body, with two subjects: astronauts Scott and Mark Kelly, identical twins. In 2015, Scott flew to the International Space Station and lived there for 340 days—a record for an American astronaut—while Mark stayed on Earth. Scientists examined the twins before, during, and after the mission.

While the study certainly detected some interesting changes in Scott after his return, space did not alter 7 percent of Scott’s DNA, the genetic code found in the cells in our bodies that makes us what we are.

Our cells have the same genes, which are made up of the same DNA, but genes behave differently; that’s how some cells produce hearts while others build lungs. The way genes are expressed can be affected by changes in the underlying sequence of DNA, by the random mutations we experience and collect over the course of our lives. But gene expression can also be influenced by all kinds of environmental factors, like stress and diet.

What the NASA study found was that some of Scott’s genes changed their expression while he was in space, and 7 percent of those genes didn’t return to their preflight states months after he came back. If 7 percent of Scott’s genetic code changed, as some of the stories suggested, he’d come back an entirely different species.  

The misinterpretation of the study’s results spread like wildfire this week, across publications like CNN, USA Today, Time, People, and HuffPost. Even Scott Kelly himself was fooled. “What? My DNA changed by 7 percent! Who knew? I just learned about it in this article,” he tweeted earlier this week, linking to a Newsweek article.“This could be good news! I no longer have to call @ShuttleCDRKelly my identical twin brother anymore.”

The stories reached a crescendo on Thursday. Geneticists who saw the news took to social media to groan about how wrong it was. The scientists behind the NASA study, stunned by how quickly and fiercely the wrong news had taken off, were bombarded with calls and emails from reporters. They scrambled to set the record straight. “I never thought I’d find myself battling ‘fake news,’” says Christopher Mason, a geneticist at Weill Cornell Medicine in New York who led the study.  (He’ll be on NBC’s nightly news on Saturday trying to explain.)

A slew of new stories eventually appeared, this time debunking what had been reported. Google News highlighted a mix of inaccurate and accurate reports. By about 4 p.m., NASA released a statement to address the mayhem and confirm that Scott did not, in fact, come back from space a mutant.

“Mark and Scott Kelly are still identical twins,” NASA said. “Scott’s DNA did not fundamentally change. 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 whole thing was a mess. How exactly did this all happen?

First, about the study. It was actually the Kellys’ idea. Before Scott launched to the space station in 2014, the pair pointed out that since they’re identical twins, maybe the space agency could study what happens when one’s on the planet and the other isn’t. So NASA put a call out for proposals and then awarded the chosen researchers a combined $1.5 million over three years. When Scott returned in 2016, scientists spent months analyzing data, looking for evidence of genetic changes that potentially could be attributed to spaceflight.

The study has one big limitation: It’s a case study of a single participant, Scott. Mark is the only control group. If scientists detected changes in Scott’s gene activity, they would have no idea whether they were due to spaceflight or any number of other factors, some as simple as just being alive for a year.

For reasons that remain unclear, the news stories that erupted this week resurfaced a couple of NASA press releases from January, which came out after the study’s scientists presented some of their preliminary results at a conference. The releases were summaries, and some of the language was ripe for misinterpretation. “Another interesting finding concerned what some call the ‘space gene,’” one press release said. “Researchers now know that 93 percent of Scott’s genes returned to normal after landing. However, the remaining 7 percent point to possible longer-term changes in genes related to his immune system, DNA repair, bone-formation networks, hypoxia, and hypercapnia.”

First of all, there’s no such thing as a “space gene.” It’s just a term that some geneticists who are studying the astronauts use to refer to genes that maybe, someday, we can determine become expressed differently solely because of spaceflight.

The rest is pretty vague and potentially confusing if you’re not a geneticist. The study’s scientists haven’t released a formal, peer-reviewed paper about their findings, which is expected later this year, so this press release was all people had to go on. The press release should have referred to changes in the expression levels of genes, and maybe tried explaining epigenetics—the study of modifications in gene behavior, and not in DNA itself. It could have even indicated that the finding wasn’t a shock.

“It’s not surprising that gene expression would change to adapt to a new environment,” says Chris Gunter, an associate professor at the Emory University School of Medicine in pediatrics and human genetics, who was not involved in the NASA study. “That’s one of the reasons that humans are absolutely amazing systems.”

Readers of the press release would have no idea whether a 7 percent change was even significant; after all, this is the first time we’ve studied gene expression in an astronaut. NASA said the change was “very minimal” in its clarifying statement on Thursday. But by then, it was already too late.

Scott did experience some changes in his DNA. DNA is packaged into structures called chromosomes, which end in protective caps called telomeres. The study found that Scott’s telomeres lengthened while Scott was in space. This was a big surprise. On Earth, our telomeres shorten as we age, and the process can be accelerated by stress. Because spaceflight puts immense stressors on the human body, scientists had thought Scott’s telomeres would shrink.

“There was no doubt that there was telomere elongation in space, and they shortened dramatically when he came back,” says Susan Bailey, a radiation cytogeneticist and Colorado State University professor who led the telomere research. But “that is very separate from gene expression. It’s not the same thing at all.”

The media seems to have conflated this finding, the lengthening of the telomeres in Scott’s DNA, with the other—the changes in gene activity. Take, for example, the Newsweek story that Kelly tweeted. “Astronaut Scott Kelly’s DNA was altered by a year in space,” it says. True, if you’re talking about his telomeres. The next sentence, “Seven percent of his genes did not return to normal after he landed, researchers found.” Also true, if you’re talking about his gene expression. Unfortunately, when they’re combined like that, they’re misleading.

There was no evidence of the deliberate or malicious spread of misinformation in this saga. But the whole affair provides a fascinating case study in the concept of “fake news.” Last week, MIT released the results of a highly regarded, massive study on the spread of false information over a decade.  The research, which my colleague Robinson Meyer has described in detail, focused mostly on false and disparaging information in political news, but it has lessons for other realms as well.  The researchers found that fake news appeared to be more “novel” than real news. For example, tweets that spread false information used words associated with surprise and disgust, while tweets that shared accurate information used words associated with sadness and trust.

We saw this play out on Thursday. “After year in space, astronaut Scott Kelly no longer has same DNA as identical twin,” that Today headline proclaimed, highlighting the novel. “No, Scott Kelly’s year in space didn’t mutate his DNA,” a National Geographic headline replied, highlighting what was real.

This isn’t even the only time news about astronauts accidentally became “fake news.” In January, the Japanese astronaut Norishige Kanai tweeted that he had “somehow” grown nine centimeters after just a few months on the International Space Station. “I’ve really shot up, something I haven’t seen since high school,” he said it. The tweet spawned many excited news stories about the strange occurrence. Less than two days later, Kanai tweeted again to apologize. He had measured himself, and it turned out that he had only grown two centimeters, an unsurprising amount since zero gravity slightly stretches the spine. “This mismeasurement appears to have become a big deal, so I must apologize for this terrible fake news,” he said.

In November, the Russian cosmonaut Anton Shkaplerov gave an interview about an experiment that involved leaving microorganisms on the exterior of the International Space Station and periodically checking on them to see how they survived. “And now it turns out that somehow these swabs reveal bacteria that were absent during the launch of the ISS module,” he said. “That is, they have come from outer space and settled along the external surface.” News organizations took this and reported that—OMG—the Russians had discovered alien life sticking to the outside of the International Space Station. Others wrote stories debunking this—but not before the news had spread across the vastness of the internet.

As news coverage of Scott and his DNA reached a fever pitch Thursday, some wondered whether NASA should have waited to release results, even preliminary ones, from the study until after their paper came out, in order to reduce the risk of confusion. “It would have been better if we could have had the paper out sooner, but we’re just trying to be very careful about this,” Bailey says, about interpreting the results. “And this is a good example of why we’re being very careful. ... We really do have some very definitive results coming out. Just wait for the paper.”

The release of preliminary results is also risky because scientists are far from making any definitive claims about how spaceflight affects human genes, as exciting as the investigation is. “What an incredible natural experiment to be able to study the effects of space on the human body. There’s a reason why twin studies are one foundational pillar of human genetics,” Gunter says. But, she adds, “we also need to acknowledge that we aren’t measuring ‘space versus Earth’—we are measuring a host of environmental components.”

The study’s investigators pretty much went in and measured everything they could find. In this kind of study, determining which factors led to which effects is nearly impossible. Nor can any results be extrapolated to astronauts or the general population. “The perfect nature-versus-nurture study would be many, many twins and many, many missions, and the challenge is we’ll probably never get that,” Mason says.

The twins study isn’t perfect, but it’s certainly a start. “One of the things I’m seeing on Twitter is that it’s an n=1 experiment, but this is kind of the reality of it,” says John Greally, the director of the Center for Epigenomics at Albert Einstein College of Medicine. “We can only do experiments on very small numbers of space travelers.”

And it’s certainly better to use twins. “If you took yourself and myself and we didn’t go to space, and you took exactly the same cells from the two of us, there would be big differences in things like DNA methylation and gene expression, which are due to our being genetically different,” Greally says. “[Mason] has been able to get rid of that one problem by using the identical Kelly twins.”

By Friday morning, most of the news stories about Scott Kelly’s DNA had been updated, their headlines tweaked and claims corrected. The storm has passed. But for a brief moment in time, many people out there, including Scott himself, believed that an American astronaut returned from a year in space with his genetic code completely transformed. It was, technically, fake news. But it was an honest mistake, not a devious campaign of misinformation.

“The way that people have run with this and overinterpreted is human nature. You don’t need to talk to a genomics person to get feedback on that,” Greally says. “But I love the fact that people care. It would be so much worse if they didn’t.”