Siblings compete; it’s pretty much a ubiquitous fact of life. So when astronaut Scott Kelly returned to Earth in March 2016 after nearly a year in space, it must have really irked his identical twin brother, former astronaut Mark Kelly, that Scott was temporarily two inches taller. However, Scott’s height was not all that changed during his time in space.

According to preliminary results from NASA’s Twins study, Scott’s year in space also drastically increased his rate of DNA methylation, the process responsible for turning genes on and off. By regulating gene expression, methylation is believed to play a major role in the development in many diseases, ranging from cancer to cardiovascular disease.

“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,” said Chris Mason, principal investigator of the Twins Study, in a press release. “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.”

As part of the study, Scott spent a year in zero gravity working on the International Space Station. In the meantime, his brother Mark went about his daily life on Earth as an author, political activist, and aerospace consultant. Taking advantage of the Kellys’ identical genetics, the Twins Study monitored subtle genomic changes that were elicited by each brother’s environment — namely Earth and space — helping researchers investigate the long-term effects of extended spaceflights.

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