Editorial use only Twins Study with US astronauts Mark and Scott Kelly. Former NASA astronaut Mark Kelly (left) and his twin brother Scott Kelly (right, both born 1964) speaking to the media about the Twins Study and the One-Year Mission. The One-Year Mission was carried out by Scott Kelly on board the International Space Station (ISS) where he was the commander for a year during Expedition 45 and Expedition 46, from March 2015 to March 2016. During this period, his twin brother Mark (who flew on four space missions between 2001 and 2011) was on Earth as the 'control subject'. The Twins Study looked at how Scott's year in space changed gene expressions compared to his twin. Seven percent of the genes that changed were still affected six months after Scott's return from his year in space. Photographed on 19 January 2015.