So, Scott Kelly’s DNA changed while he was in space:

Scott Kelly has spent over 500 days in space overall, but a huge chunk of that came with a single mission which had him stay aboard the International Space Station for 342 days. His brother Mark, who is a retired astronaut, is his identical twin and has the same DNA. This provided a never-before-possible opportunity for NASA to study how long-term space travel affects the human body and the genes that make us who we are. As it turns out, space really does change us, and upon Scott’s return to Earth it was discovered that his DNA has significantly changed.

“Scott’s telomeres (endcaps of chromosomes that shorten as one ages) actually became significantly longer in space,” NASA explains. “While this finding was presented in 2017, the team verified this unexpected change with multiple assays and genomics testing. Additionally, a new finding is that the majority of those telomeres shortened within two days of Scott’s return to Earth.”

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Another interesting finding concerned what some call the “space gene”, which was alluded to in 2017. Researchers now know that 93% of Scott’s genes returned to normal after landing. However, the remaining 7% point to possible longer term changes in genes related to his immune system, DNA repair, bone formation networks, hypoxia, and hypercapnia.