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.