My Twitter feed was buzzing this week with reports that Scott Kelly’s genes were knocked permanently out of phase because he spent a year in space.

Some of the reports made it sound as if Scott Kelly was no longer the identical twin of retired astronaut Mark Kelly, who participated in the genetic study down on Earth.

Scott referred to that claim in a tweet that was aimed at tweaking Mark — as brothers are wont to do.

What? My DNA changed by 7%! Who knew? I just learned about it in this article. This could be good news! I no longer have to call @ShuttleCDRKelly my identical twin brother anymore. https://t.co/6idMFtu7l5 — Scott Kelly (@StationCDRKelly) March 10, 2018

Mark Kelly played along:

I used to have an identical twin brother. Then this happened…. https://t.co/zQAKU4Nbji — Mark Kelly (@ShuttleCDRKelly) March 15, 2018

The truth is more complex. The whole point behind the DNA comparison was to see how the effects of long-term spaceflight, including exposure to space radiation, affected the normal patterns of mutation and gene expression. (Because of accumulated mutations, the complete DNA codes for two individuals are not identical, even if they’re identical twins.)

Preliminary results have shown that Scott’s genes did undergo significant changes — but many of those were shifts in gene expression, due to environmental factors such as oxygen deprivation stress, increased inflammation and changes in diet. If genes can be compared to light switches, then gene expression has to do with which switches are turned on or off.

Another interesting change affected Scott’s telomeres — the structures on the ends of chromosomes that usually shorten as people age. While Scott was in space, his telomeres actually lengthened.

Most of the changes, including the lengthening of the telomeres, faded away after Scott’s return to Earth in March 2016. But some of the changes in gene expression have persisted. That’s what sparked the buzz, and the Kellys’ banter.

The buzz got so bad that NASA stepped in today with a clarifying statement:

“Mark and Scott Kelly are still identical twins; 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 change related to only 7 percent of the gene expression that changed during spaceflight that had not returned to preflight after six months on Earth. This change of gene expression is very minimal. We are at the beginning of our understanding of how spaceflight 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.”

The good news is that those comprehensive results should provide new insights into how trips to the moon, Mars and beyond could change the inner workings of astronauts’ cells.

The bad news, at least for the Kelly brothers, is that they’re still twins.

For more about the space-gene brouhaha, check out this reality check from Ars Technica’s John Timmer.