After a yearlong mission in outer space, Scott Kelly came back a changed man — genetically, "Today" reported.
A NASA study found that the 54-year-old no longer shares the exact same DNA as his identical twin, fellow astronaut Mark Kelly.
According to NASA, 93 percent of Scott’s DNA returned to normal after landing, but seven percent did not. This can point to possible longer-term changes in genes related to his immune system, DNA repair, bone formation networks and more.
“I no longer have to call @ShuttleCDRKelly my identical twin brother anymore,” Scott joked on Twitter.
Editor's note (March 16, 2018, 3 p.m. ET): NASA clarified that Scott Kelly's DNA did not fundamentally change, but that his gene expression did, noting that "this likely is within the range for humans under stress, such as mountain climbing or SCUBA diving."