In addition to the biological tests conducted on all astronauts on the station, Kelly also participated in comparative study on the effects of spaceflight with his identical twin Mark as the ground control subject. Kelly's physical, cognitive, and genetic traits were measured before and after the flight. Within several months after returning to Earth, Kelly had adapted to living in gravity. Genetic tests revealed changes in Kelly's gene expression, and an increase in the length of his telomeres relative to before his flight.