You might remember the NASA twins study in which 10 different teams of researchers studied astronaut Scott Kelly, who spent a year aboard the International Space Station in 2015 and 2016, and compared him to his identical twin, fellow astronaut Mark Kelly, who had remained behind on Earth.
Part of that research involved studying and comparing the twins' DNA, and as this Atlantic article details, it created a stir when numerous news outlets misinterpreted the results and reported erroneously that Scott Kelly's DNA had been altered significantly by his time in space. In reality, as this NASA media release explains, Scott Kelly's DNA didn't fundamentally change. But researchers did observe changes in gene expression — that is, how genes react to the environment.
Most of Scott's gene expression returned to normal after he landed back on Earth, but 7 percent of his gene expression didn't revert. As the NASA release notes, that 7 percent difference points to possible longer-term changes in genes related to Kelly's immune system, DNA repair and bone formation networks. The changes might also affect how Kelly's genes respond to hypoxia, or oxygen deprivation, and hypercapnia, the condition of having too much carbon dioxide in the bloodstream. (The latter is a potential problem on the ISS, where, as this 2012 study notes, ambient CO2 levels rise above normal atmospheric conditions on Earth, and crews routinely report symptoms such as headaches and lethargy.)