NASA has just made a remarkable finding, thanks to research they conducted on two amazing twin brothers, Mark and Scott Kelly. As Scott Kelly spent time on board the International Space Station, his twin brother Mark was here on Earth and monitored by NASA, and now that Scott has returned the agency was able to compare their DNA to further understand the effects of space on the human body.

NASA has been running a lot of tests ever since Scott Kelly returned to Earth, and scientists were stunned to find that space actually changed Scott’s genes as compared to his brother. Specifically, it altered the process that regulates gene expression called methylation.

Methylation, which is the process by which genes are switched on and off, was sped up in Scott Kelly’s body during his time in space. You can watch video from NASA describing the process, which is embedded at the bottom of this post.

“Some of the most exciting things that we’ve seen from looking at gene expression in space is that we really see an explosion, like fireworks taking off, as soon as the human body gets into space,” Twins Study Principal Investigator Chris Mason, Ph.D., of Weill Cornell Medicine, said. “With this study, we’ve seen thousands and thousands of genes change how they are turned on and turned off. This happens as soon as an astronaut gets into space, and some of the activity persists temporarily upon return to Earth.”